Svchost.exe is the most mysterious process in Windows XP / Vista. Svchost.exe is a generic host process name for services that run from dynamic-link libraries (DLLs). The authentic svchost.exe file is located in C:WindowsSystem32, but a lot of viruses and trojans use the some file and process name to hide their activities.
The free Svchost Process Analyzer application will enumerate all svhost instances and checks the containing services. So it is easy to discover svchost worms like the infamous Conficker worm.