Disabling services to increase performance is a MYTH. SERIOUSLY. DONT BELIEVE ANY "Service Tweak guides" on the internet, LEAVE THEM AS DEFAULT.
Windows 5.x [Windows XP and Windows 2003] have inferior CPU Priority Scheduling compared to the new Windows 6 [Vista/2008] family. I have done a LOT of research on this, and my own tests. And I have a very slow computer - a P4 with only 1GB of RAM. Disabling EVERY service that wasnt critical provided NO performance increase in gaming compared to the default service configuration. Windows 5 is sh*tty and will benefit from disabled services, but Windows 6 is uber l33t and has very VERY impressive kernel-level thread prioritization. It is the single reason why I refuse to go back to XP
With that said, there may be some services that you actually DO want to disable for non-performance reasons. Windows Update for example to sure-shot block Auto Updates, or the DOMAIN-SERVER related services because you are 101% sure they will never be needed. But I highly suggest that you leave the defaults for maximum compatibility - I made the mistake of disabling things like "Application Compatibility" among others and half my programs crashed randomly or InstallShield programs [90% of games] would fail installing.
If you don't know much about services, I highly suggest you just leave it. If it aint broke, don't fix it.