If that's the exact error message (...IsThreadDesktopComposited...)
then the most likely cause is that some program has erroneously installed files intended for Windows Vista or 7 only under WinXP.
Go to your "C:\Windows\System32" dir
Look for this file:
If it's there, either rename it to DWMAPI.DLL.OFF or move it out of the Windows dir to some backup folder elsewhere (don't delete it just yet, just incase).
DWMAPI.DLL shouldn't even exist on WinXP, as it's part of Vista's Desktop Windows Manager.
Now see if you get that error message when opening Winamp and other programs.
If it's not even there, or if the error still occurs,
then it means some other WinXP system file(s) have been wrongly replaced with the Vista version...
and you'll need to use Dependency Walker
to try work out what files need restoring.