Win XP Pro at work, Win 2000 Pro at home, though mainly for games and browsing. But if I had to pick just one to work with, it would be Linux. I'm a programmer by trade and mainly do Java, so I can use just about any platform for that. I've had some Macs in the past and like them just fine, though I haven't used one since OS X came out, so it's been a while.
Started out with a TRS-80 Color Computer and later the Apple ][. First PC was an IBM PC Portable (it had a handle, but weighed something like 40 pounds!) with 256K (NOT Meg) and dual 5 1/4" floppies. No hard drive. DOS 2.1.