One thing you can do to speedup the shutdown is to avoid having so many processes start in the first place. This is probably more useful for startup, but it might help shutdown as well.
1. Go to start -> Run...
2. type "msconfig" (no quotes) and press enter
3. Go to the startup tab (I think that's what it's called) and uncheck programs that you don't need starting. For example, I don't need iTunes stuff starting, I don't need Quicktime crap starting, I don't need Adobe Reader stuff starting, etc.
4. Save, reboot, and when it boots up again and gives you a message warning you that the system has been changed, click the checkbox that says not to alert you anymore.
If it's taking a really long time to shutdown, perhaps some process is hanging or something. I don't know, but the above info is at least one tweak you can make to speed it up. You might also look at increasing the size of your virtual memory as well. Not sure that either one will help shutdown speeds, but should improve performance in general.
P.S. Your sig says "Windows Vista RULES" but you're saying that you have reinstalled XP? ROFL!