These days, computers are NEVER really idle.
Windows Xp has a minimum of 11 processes running in the background at all times. If you have antivirus, you'll have probably 15 processes.
Then add processes and services for your video card, sound card, mobo drivers, adobe download manager, any P2Ps you may have, and a few other update agents....
Before you know it, you have 50-60 processes running in the background, and your computer is never really idle.
And if that doesnt bug you, get Vista it NEVER stops "thinking."