I saw your recent review of ICE, the emergency medical information app for the iPhone. But my iPhone is set up to require a passcode to get to the home screen and run apps. Is there any way to display emergency contacts before a person enters the passcode?

There are some apps that allow you to customize the wallpaper of your iPhone or iPod Touch by adding a few lines of text that can be seen before it’s unlocked. One, called Close Call, displays a red cross and, next to it, an emergency phone number, and one brief line about, say, a key allergy or medical condition. The app is free, and is from a company called Polka, at polka.com.

I installed VMware Fusion on my Mac so I can run a virtual Windows XP computer on the Mac. When in the virtual Windows environment, I use security software. If I have the Fusion program shut down completely when I am not using it, will my computer be vulnerable to Windows security threats?

The vast majority of viruses, spyware and other malicious software is designed to run on Windows and cannot run natively on the Mac’s built-in operating system. So, while you are wise to use security software in your virtual Windows PC, any malware it misses won’t be able to run when Fusion is shut down. While even a virtual PC can get real viruses, this faux PC just disappears when Fusion is shut down, and so does all its software, including viruses.

But simply because Windows-based malware can’t run on the Mac’s OS, that doesn’t mean you don’t have to be careful. You can still be tricked by false email and Web sites into giving up personal information.

• 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 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.