OSX86 has been around for a while. It allows you to install OSX on a regular “PC” (Macs ARE regular PC’s, regardless of what Apple will lead you to believe). But what if you are not ready for that level of commitment?
Enter the wonderful world of virtualization: Lifehacker has a great guide right here.
Running Windows on a Mac has been trivial for a long time – even sanctioned by Apple themselves. Personally I have been running windows via Parallels on my MacBook for a while, but I now bootcamp into Windows 7 on a daily basis. I still think that Apple has the edge in terms of usability, but Windows is getting awfully close to nipping OSX in the butt.