Here are a few questions about computers I’ve received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability.

In last week’s Mailbox, you said that Windows XP machines can slow down over time unless you do “a lot of techie maintenance.” What did you mean? I regularly defragment the hard disk, tweak the registry, and clean out temporary files, but my PC is still slow.

All of these things are helpful, though I don’t recommend that nontechie users “tweak” the Windows registry, which contains vital program information that can mistakenly be removed or altered if you don’t know what you’re doing. Another speed-enhancing tactic is to use a program like Startup Cop Pro (snipurl.com/b4v91) to prevent unneeded programs from running at startup, and one like PC Decrapifier (pcdecrapifier.com) to clean out craplets — unwanted pre-installed programs.

However, what I had in mind when I wrote that was something more drastic, something a number of techies I know do annually: a complete replacement of Windows. This involves first backing up all your files, and then performing what’s called a “clean install” of Windows XP that wipes out everything on the PC and starts fresh. You then would copy back all your data files and re-install your programs.

This can make the computer feel like new, but the problem is that it can be tricky and tedious for nontechie users. Depending on the source of the copy of Windows XP you are using for the clean install, you may have to locate and re-install drivers for peripheral hardware and for hardware features of your particular make and model of PC. You may run into licensing and activation issues with your re-installed programs. And you may have to download numerous patches and upgrades for Windows itself and for your programs.

I am considering buying one of the new MacBook Pro 15″ laptops. What do you think of the keyboard on this laptop? Is it easy to type on compared to other laptop keyboards? What do you think of the shiny screen?

Because keyboard and screen preferences vary from person to person, I strongly urge all laptop shoppers to try out models they are considering before buying, even if only for a few minutes at a retail store. Having said that, I find the MacBook Pro keyboard to be very comfortable and easy to use, with good key spacing and feel. I personally prefer matte screens to glossy ones, but own laptops with both types and find the glossy ones acceptable, if not optimal.

I use the Windows Mail program that came with Vista on my computer. You say Windows 7 won’t come with that program. Is there something similar that can be installed?

Microsoft will encourage people to download a similar free program called Windows Live Mail, which is closely tied to its Live online service. Or, you could switch to a competing email program, or rely on Web-based email.

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.

A friend of mine suggested that I “defragment” my computer every once in a while. He says it will boost performance, but he also said it will move my data around. I tend to be very organized as to where I put documents on my computer. Will defragmenting my computer jumble my files and put them into random places?

No. Defragmenting a hard disk doesn’t change or scramble the organization of files and folders that you have established. Your file organization should look just the same after defragmenting the disk. What defragmenting does is rearrange the location on the disk of the data segments that make up your files. In some cases, a single file like a word-processor document might actually be made up of multiple data segments, invisible to you, that are located in widely separated physical spots on the disk. Defragmenting tries to optimize the location of these segments so that the computer operates more efficiently.

I saw your review of Norton Internet Security 2009, and wonder what is the difference between this product and Norton 360?

Norton 360 has the same features as Norton Internet Security, but adds some additional ones, like online backup and PC “tune-up.” It also costs more. However, the current version of Norton 360 doesn’t include the improvements I wrote about in the NIS product that make it faster and less of a burden on your computer. Those are planned for a future version of Norton 360.

Can I use one of the new 3G iPhones as a wireless modem for my Apple laptop?

No. That function isn’t built into the iPhone and Apple yanked from distribution a third-party program that enabled using the phone as a modem. I suspect the reason is a business one, not a technical one. Cellphone carriers like AT&T typically take the position that connecting a laptop to their networks, whether via a data card or via a cellphone used as a modem, should command a higher monthly data charge than they typically levy for a cellphone alone.

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