This laptop has Windows XP Pro dual booting (one boot for gaming, the other for general use).
My other PC is constantly being formatted and changed, but usually has anything ranging from Windows 98, Windows 2000, Windows XP, Windows Server, and some variant of Linux, in all weird and wonderful configurations.
It's just for testing purposes, I can get the OSes free from college to help me in my course you see (before anyone starts in about them not being legal).