Windows XP is bad ass, but it lacks a feature that's in vista and that feature is even better on Windows 7. I've noticed this because, when I used XP for a while, and I know how to keep a computer clean, no matter what I do, it will slow down in the end. With Windows Vista, (On the same computer) This happened much slower, and for Windows 7, I've tried the new OS, and Windows 7, I got to say, is way faster than Windows XP.
I would give Windows XP a 8.5/10
Because of Windows Vista's "harder to get slower" feature, I give it a 7.5/10
Windows 7 no doubt gets a 9/10