Sometimes there's a problem when a service running under svchost.exe grabs 100% of the processor (Two programs are hogging my processor, below). This can get people to investigate the whole services area, where they would not normally venture....
Microsoft provides a program to control Windows Services. To run it, go to Start|Programs|Administrative Tools and select Services. This lists all the services, tells you what they do, and tells you their status.
Select a service and double click it. This pops up a properties sheet. Around the middle is an entry that says Startup type. Click the drop-down arrow and this enables you to select Automatic, Manual or Disabled for that service.
You can do a lot of damage this way ;-)
The Elder Geek has a good account of the system in its Services Guide for Windows XP.
