If you have some old Windows 98 games that you love replaying, then I would suggest Windows XP 32-bit, as it has better compatibility with older games.
Other than that, go with Windows 7. It's better, the new Explorer and Start Menu Search look and work similiar to OS X's Finder and Spotlight and it's more secure than XP (which is coming up on its nineth year). I've never used Windows XP on a Mac but I use Windows 7 on my MacBook Pro and it works beautifully. I'm going to stick to Apple laptops to run Windows on from now on.