I currently use an old laptop running XP to do platform testing, but with the new Intel based macs there is a program available that lets you run XP like any other application, a sort of cross between Virtual PC and Bootcamp. I saw it in action the other day but can't remember what it's called though. Looked really impressive.
Here's something I came up with via Google that you might be able to use:
http://websitetips.com/tools/compatibility/