The ability to run the Microsoft Windows OS is one of the most amazing innovations on the Apple platform. Until last year, users making the switch from Windows had to kiss their old software goodbye. Now (assuming you have a new system equipped with an Intel processor), you can run Windows applications on the Mac, which is astonishing to anybody who's been around personal computing for a while.

There are four major commercial options for running Windows on the Mac: Parallels Desktop from Parallels; Fusion from VMware; CrossOver Mac from CodeWeavers; and Apple's own Boot Camp.

Parallels Desktop and VMware Fusion are both virtual-machine software that run the Windows operating system on top of the Mac OS, and Windows applications on top of the Windows "guest operating system."

	(click image for larger view) Parallels lets you run Windows apps on a Mac: Here you see Internet Explorer running on Windows XP on the Mac. view the image gallery

Parallels lets you run Windows apps on a Mac: Here you see Internet Explorer running on Windows XP on the Mac.

Early versions of Windows virtualization software required you to take a "window-in-a-window" approach -- in other words, you ran the Windows application inside another window containing the Windows desktop. Either that, or you replaced your entire Mac desktop with a Windows desktop. That was awkward.

More recently, both Parallels and Fusion can be configured to give each Windows application its own window, so it looks somewhat like a native Mac application. You can get a good idea of that by looking at the screenshot of Parallels on this page -- it looks like Microsoft Internet Explorer for Windows XP is running on the Mac desktop. You also can see the Windows taskbar on the bottom of the desktop, and the Mac menu bar on the top. Parallels calls that feature Coherence; VMware calls it Unity.

Both Fusion and Parallels support minimizing Windows applications to the Dock, or spreading them out on your desktop with Exposé.

Parallels came out last year; I've been using it occasionally for several months, mostly when I need to access a Web site that requires Microsoft Internet Explorer. Others use Parallels to run Office for Windows -- while Microsoft has a version of Office for Mac, it's a couple of years out of date. Office-on-Windows-on-Parallels users say the Windows version is faster, even running on Parallels, and easier to use than the native Mac version.

Parallels Desktop for Mac is priced at $80; VMware Fusion is priced at $60.