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 bought an iPhone and was frustrated that my fingers appear to be too large to type properly on its virtual keyboard. Is there a stylus that works with the iPhone?

I hadn’t realized this until recently, when a friend who bought one showed it to me, but, yes, there is. The iPhone wasn’t designed to operate with a stylus, and the small, thin, hard kind that one uses with, say, a Treo, doesn’t work right with the iPhone. But there are some companies that sell a wider type of stylus with a soft tip that approximates a small fingertip, and is designed for the iPhone.

If you type “iPhone stylus” into a search engine, or an online store like Amazon.com, you will see a number of inexpensive choices. One downside: the iPhone doesn’t have a built-in slot for a stylus. My friend carries his, which has a clip on the end, like a pen, in his pocket.

I just purchased an external hard drive for my PC that connects through a USB port to back up photos and important files in the event of a computer crash. Would this hard drive be affected if my main hard drive crashed? Should I disconnect the external drive when it’s not in use?

The two hard disks are separate devices, and if one has a mechanical failure, it doesn’t affect the other. There might be some scenarios in which a problem with the computer itself, as opposed to a breakdown of its internal hard disk, could affect the external drive. And malicious software could corrupt or erase files on the external drive. An electrical surge could also affect both drives, or fry the computer itself.

Many people can’t disconnect their external drives, because they use automated backup programs, or frequently save files to the drives. If you are doing only occasional manual backups, you could disconnect the extra drive when not in use, so you’d feel more comfortable. But, for maximum peace of mind in case of an electrical surge, you should disconnect it not only from the PC, but from the electrical outlet as well, even if you are using a surge protector.

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