I have multiple computers. Windows XP SP2 is currently my main OS since it's installed on my main laptop. The system restore disc only has SP2 installed (since SP3 wasn't around back then). I'll probably be updating it to SP3 soon.
My other computers use XP SP3, Windows 8, and Windows 7 (it's a netbook so it's the starter edition
). I've used pretty much all of the Windows OSs including 95, 98, 98 SE, 2000, Me, XP, Vista, 7, and now 8. XP is by far my favorite out of these.
I also have a few Linux discs which I boot from every now and then. I mostly use Ubuntu or Linux Mint but I also have discs for Knoppix, Morphix, Browser Linux, and Puppy Linux.