Apple Boot Camp let's you run both Leopard and Windows on your Mac, but Boot Camp’s biggest drawback is its requirement that you reboot your computer every time you want to switch between Leopard and Windows. Two $80 remedies are readily available, Parallels Desktop from a Virginia startup, Parallels, Inc., and VMW are Fusion from VMware of Palo Alto, California.

Their respective software takes the form of a virtual machine. The programs simulate a Windows machine inside its own screen within OS X. Or, if you feel like it, you can go full-screen with Windows. The fake machine behaves just like the real deal. You can add software, surf the Web, listen to music, and play Windows games on a Mac.

You can even apply this virtualization stuff with versions of Windows dating back to Windows 3.1 as well as Linux, Solaris, OS/2, MS-DOS, and other operating systems.

Parallels and Fusion differ from Boot Camp because you can run any OS while you run Leopard, without having to restart. What’s more, you can share files and folders between OS X and Windows and cut-and-paste between the two. The Coherence feature inside Parallels lets you run Windows programs like they were Mac apps.

Neither program, as of this writing, supports the prettier Aero interface in Vista, and installation can be a bit tricky. But if you’re interested, check out Parallels and VMWare Fusion.

Virtual or not, you’re running Windows on or inside your Mac. So take all the usual precautions by loading antivirus and other security software.