A while back there was a legendary site by BlackViper (you can still find mirrors of that site by Googling 'BlackViper) with tweaks to enhance XP by editing the services.msc section of that OS to cut down on overhead and provide better performance than the default values gave you.
Well... it seems Viper is back, or at least a close relative in this new site:
http://www.speedyvista.com/services.html
This is without a doubt the best reference I have found for Vista tweaks to date, and having implemented some of these, I can say, I'm impressed.
Totally comprehensive, and hopefully the site will be updated with more commentary.
Bookmark this site!!