I am a Mac user too, and face the same issues.
Maybe not want you want to hear, but if you do lots of web design, how about picking up a small, used Windows laptop on eBay?
That may sound extreme, but considering you can buy new computers for $600-$800, you might be able to find something that is small for a couple hundred dollars.
Of course if you have a need to test on XP to Windows 8 (or whatever) then any way you go will cost more money and take more time to set up.
Here is what a quick search brought up...