Eek!
You may want to hide your children from this one. Scaaaary stuff:
For my current work, I needed to be able to look at behaviors & metrics of a PC app while developing the Mac OS X version of it, and using Virtual PC was preferable to actually having to struggle with a real, live Winders box. Still, it scares me. I now have to think about viruses and patches and the fact that the colors are so garish in the XP user interface that when I switch back to my lovely, elegant, easy-on-the-eyes Mac OS X apps, there's an after image burnt into my retinas. I honestly don't know how you Winders folks do it.