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.

I have moved from a PC to the iMac. In the Windows environment, I felt a need to run utilities to clean out the registry and defragment the hard disk frequently. Is this also needed on the iMac? If so, what programs are recommended?

The Mac operating system, called OS X Leopard, doesn’t include a registry, which is a feature of Windows that holds information that programs need to operate properly. So there’s no need to clean or maintain any registry on a Mac.

Mac hard disks, like those on Microsoft (MSFT) Windows computers, can get fragmented — a condition in which parts of files are so scattered around on the disk that the disk runs slowly. However, the operating system has some under-the-covers features that generally obviate the need to run a defragmentation utility. In fact, Apple (AAPL), which calls defragmenting a disk “optimizing” it, flatly claims that “You probably won’t need to optimize at all if you use Mac OS X.” There are some Mac defragmentation utilities, but I don’t believe you will need them unless you have large numbers of extremely large files and almost no free disk space.

My son’s computer frequently gets infected with adware, pop-ups. Recently it was hit with a continuing pop-up ad called VirusHeat that touted itself as a solution to the computer’s problems. When I paid for VirusHeat, the problems went away. Is it legitimate?

According to numerous reports on the Web, including some from security companies, VirusHeat is a form of malicious or misleading software. It falls into a category that attempts to scare people into thinking their computers are badly infected, or exaggerates any problems you may have. This is a common tactic now used by creators of malware.

Some of these fake or misleading “security programs” may be designed merely to make you pay. Others may even be designed to install the very kinds of viruses, spyware or adware that they claim to fight.

I have updated to a new PC. My data are on a floppy disc. There is no floppy disc drive on this new computer. How can I transfer my data?

For around $25, you can buy an external floppy disk drive that plugs into a new PC using its standard USB port. If you do so, and connect it to the new PC, you should be able to copy your data to the new computer’s hard disk.

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