Right now I have 3 Windows Explorers, WhatPulse, Internet Explorer, Firefox (with 7 tabs open), Hamachi, Google Talk, uTorrent, WinAmp, Microsoft Groove, FileHamster, Active Desktop Calendar, PowerISO, UltraMon, and Symantec AntiVirus running or in the tray. With a total of 55 Processes running.
My System Idle Process is usually at 98 or 99% with an occasional dip down to the mid 80s for about a second.
I don't know how it's possible to get 20% CPU usage when you're not touching anything.
EDIT: My CPU is a single core AMD 64 3500+ 2.2Ghz running on WinXP SP2 (32-bit)