What is svchost.exe And Why Is It Running? - How-To Geek
Notice the important line about disabling un-needed services... then check this out:
Windows 7 Services that can be Safely Set to MANUAL - Tweaking with Vishal
Then follow up with:
http://www.theeldergeek.com/services_guide.htm (even though it's for XP, there's still a lot of relevance)
Edit: Not sure about Windows 7, but you might have to run services.msc to see all the services represented in the elder geek thing.