XP Home (maybe switching to Pro, I can get it fully licensed for free through my University). I'm a power user, I want Pro!
Knoppix for system recovery (but it hangs my laptop).
Ubuntu Live CD (won't even load on my laptop, works on desktop PC).
I've got a crappy old rig with Win 2000 (used to be 98SE) and Fedora Core 3.
I've got an even older, even crappier rig with Win 3.1.
And 10 years ago I had a machine with Win 95.