A: You will begin to see more touch-screen laptops after Oct. 22, when Windows 7 comes out, because that new operating system has optional fingertip touch-screen navigation built in. In addition, many of the major Windows PC makers are adding their own touch-navigation systems to newer models.

However, I’d offer two caveats. First, not every touch screen will be capable of performing all the multitouch actions and gestures Windows 7 will offer. Some are more limited in their capabilities.

Second, the hardware makers will likely be cautious at first about adding fully capable touch screens to all or most of their laptops.

This is partly because these screens can add significant cost in a price-conscious market, and partly because nobody is certain how popular touch-screen navigation will be on laptops as opposed to, say, on all-in-one desktops where reaching for the screen is more obvious and natural.

I have an Apple iMac, and an Apple iPhone with around 122 apps. My question is, why don’t these iPhone apps work on the iMac? Both units use the same operating system, so why not?

A: While the iPhone and the iMac each use a version of the same operating system, Apple’s OS X, they aren’t the same operating systems. The version on the iPhone is stripped down, and has been modified to support an entirely different user interface, navigation system and set of hardware capabilities.

So there are two separate collections of apps, or programs, that run on the two platforms.

I am currently running Windows Vista Ultimate on my Dell PC. I have regretted it ever since I upgraded from XP because of its terribly slow performance. I have heard good things about Windows 7 but I am afraid I will have to buy the more expensive Ultimate upgrade (which I don’t really need) instead of an upgrade to Home Premium or Professional. Is that true?

A: According to Microsoft, owners of Vista Ultimate can only perform an “in-place upgrade” to Windows 7 Ultimate, not to lesser versions. An “in-place upgrade” is the type most people think of, the type where all your programs and files and settings are preserved just as they were, but you emerge with the new operating system.

Even then, you must be careful to buy the edition of Ultimate that matches the type of architecture your machine possesses, either 32-bit or 64-bit.

However, you can downgrade to the Home Premium or Professional versions of Windows 7, if you are willing to do a lot more work, by opting for what Microsoft calls a “custom installation,” something commonly known as a “clean install.” This typically means, first, backing up all your files and settings to an external hard disk, then allowing the Windows 7 installation disk to wipe your hard disk clean before installing your preferred version of Windows 7.

You would then copy back all your files. But you cannot do the same with your programs. For these, you would have to reinstall every one, from their original disks or downloaded installation files, and then download and re-install all the patches and program upgrades that have been issued by their makers since you acquired them.

You can find Mossberg’s Mailbox, and my other columns, online for free at the All Things Digital Web site,

http://walt.allthingsd.com.

At work I am told to turn off my desktop PC at the end of the day. My wife’s office tells her to leave her computer on overnight. My former employer said to turn off the monitor but leave the PC on. What’s right?

Given the energy, environmental and economic crises, my view is that you should completely shut down both your computer and your monitor any time they will be unused for more than a few hours. And, of course, you should set them to go into sleep mode after they are idle for far less time than that. To be honest, I myself am not always faithful to that advice, but I am resolved to get better at it.

Of course, there are some nuances to the issue of overnight shutdowns. If you, or your company, routinely perform important operations overnight, like backups or security scans or system tune-ups, then you have to leave the computer on, but you should still turn off the monitor.

How much memory do you recommend for a new computer running Vista Home Premium?

Two gigabytes minimum, and three gigabytes if your budget can handle it. The latter is roughly the maximum that popular consumer versions of Vista (called 32-bit versions) can use.

For smooth, quick operation in common tasks, it’s been my experience that Vista demands more memory than either Windows XP or Apple’s Leopard operating system. And the need for added memory rises if your PC uses “shared” video memory, which means your graphics system lacks its own dedicated memory and relies on draining the main memory. This arrangement is common in less expensive computers.

We are planning a trip to the British Isles this summer with a side trip to Brussels. We plan to bring our iPhone on the trip. What is the current situation with using European networks? Will my U.S. AT&T account just work or do I need to make pre-arrangements? I realize my iPhone won’t be able to access the faster 3G networks over there, but can I use slower networks? Being limited to Wi-Fi would be a bummer.

The iPhone 3G can indeed handle 3G networks in Europe, though the original iPhone can handle only slower networks. Both models can use Wi-Fi there.

You will need to call AT&T before you leave, though, and “provision,” or set up, your account, for international roaming. Note that such roaming is expensive, with very high rates for both voice use and data use. AT&T has some monthly add-on plans that cut these fees, but, if I were you, I would turn off data roaming until and unless I needed it; do most email and Web surfing via Wi-Fi; and consider downloading Truphone, a voice-calling program that uses Wi-Fi instead of cellphone networks.

You can find Mossberg’s Mailbox, and my other columns, online free of charge at the new All Things Digital Web site, http://walt.allthingsd.com.

I read your recent column about running Windows on a Mac. Is there a way to run the Mac OS X operating system on a PC, like a Dell or a Sony?

Yes, but it is really only for tinkerers and hackers, and even then, it may not work fully or smoothly. Apple makes the process difficult, both technically and legally, because, unlike Microsoft, it is not in the business of selling its operating system for use on other companies’ hardware.

Therefore, I know of no simple software for average consumers that can install OS X on a non-Apple computer without any technical knowledge and with full assurance that the operating system will be completely compatible with the hardware. There is nothing on the market like the products that allow people to run Windows on a Mac.

A small number of techies do install Mac OS X on non-Apple PCs every day, and some people have created software to make this possible for people with plenty of technical knowledge. There is even a company called Efix that is selling a hardware module that it says will make this easier. Its maker warns the product is only for “enthusiasts” and it requires that you attach it to the computer’s internal circuitry. Also, it only works with certain types of personal-computer hardware.

One more thing: Apple takes the position that its licensing terms limit the use of OS X to Apple hardware, so, even if you can pull it off technically, there could be legal jeopardy involved. In fact, Apple is suing a company called Psystar that sells non-Apple PCs with OS X pre-installed.

I have a new PC that came with a 64-bit version of the Windows operating system. It gives me a choice between using a 32-bit Internet Explorer or a 64-bit Internet Explorer. Which should I use?

The 64-bit version of Windows, which is rarely used by average consumers, can make the computer faster, but only when running programs that have been written in special 64-bit versions. In the consumer arena, there are too few such programs, and thus too little benefit, to justify paying extra for 64-bit machines. In fact, most people who have computers running 64-bit Windows are mostly using older 32-bit programs, which run fine, but aren’t made quicker.

So, the 64-bit version of Internet Explorer may run faster. But there is a downside. Because of the relative rarity of 64-bit users, some browser add-ons and toolbars and some of the Web technologies that power the features of Web pages, aren’t compatible with the 64-bit version of IE. On top of that, you may not notice any huge speed difference as the perceived speed of Web browsers depends more on the speed of your Internet connection than anything else. So, for now, I would stick with the 32-bit version, for compatibility’s sake. You can always install the 64-bit version later, if you decide that the Web sites you frequent and the add-ons you use work well with it.

I am running a Mac with Windows XP Pro in Boot Camp, which requires rebooting to change operating systems. Is there some way I can use my already-installed Boot Camp setup as a virtual machine using VMWare Fusion, so I can run my Windows and Mac programs side by side?

Yes, there are two ways. First, Fusion allows you to treat the Boot Camp section of your hard disk, called a “partition” — which is essentially a separate Windows PC stored on your Mac — as if it were a “virtual machine.” This approach still allows you to reboot the entire Mac into Windows when you’d like to do so, in order to perform the few tasks that Fusion can’t handle, such as advanced 3D graphics.

The second approach is to simply import the Boot Camp installation and turn it into a purely virtual Windows computer, as with any virtual Windows machine you would create in Fusion. If that meets your needs, you can then actually remove your original Boot Camp Windows installation, which will likely free up some hard disk space. To do this, just select your Boot Camp partition, which Fusion automatically detects, and click on Import under the File menu.

You can find Mossberg’s Mailbox and my other columns online free at the All Things Digital Web site, http://walt.allthingsd.com.

Write to Walter S. Mossberg at mossberg@wsj.com