I did a big patchup of my computer a week or so ago: amongst it all was Windows service pack 2. I was a bit surprised to find that the system slowed down a fair bit (initially). It seems to have picked up a bit now, but I:
* Stopped windows firewall / updates / virus scanner monitor (from the new fan-dangled "security centre")
* Disabled the security centre monitoring / reporting - I do it all myself.
* Deleted the contents from the windows\prefetch and window\temp folders. Apparently this helps.
* Stopped (and disabled) a lot lot of useless (for me anyway) services
* Removed several items from the control panel
* Got rid of heaps of old programs that were just taking up space / memory.
It's running pretty smoothly now. Yay.