IIRC the latest version of Catalyst doesn't run on XP.... but I could be wrong.
11.7 still aviable for XP, no word from AMD about to stop supporting it...
probably when Microsoft does. would make sense.
ATI did relased the last Win2k compatible driver (6.2) after Win2k's EOL...
Wikipedia:
The Windows 2000 family of operating systems moved from mainstream support to the extended support phase on 30 June 2005.