svchost.exe is the exe file that runs many of the windows services that makes windows appear like it does. One of the svchost.exes are maintaining the XP themes (if you disables it you'll get a classic Win 200x look with no option to change themes). It does also include the help system and indexing services, as well as network services.
Start -> Run -> services.msc
And you'll get a list of services you can disable to improve system speed.
http://www.windowsnetworking.com/kbase/Win...erformance.html
Here is a list of "safe" things to disable. At least things most of us won't miss.