Where I work older single-core desktops go from 0-5% all the way up to 100% CPU usage when their antivirus software is being updated. Additionally, most operating systems have background tasks (basically, just housekeeping) and those tasks do require CPU cycles.
Just out of curiosity... what is your processor and operating system?
Pentium D @ 3.0ghz. I heard lots of complaints about it, but as for me; it runs very solid, even in 3D games. Multitasking, it struggles... But I heard complaints of heat, I got a coolermaster on it. Runs at 35C idol
As for the OS, Windows Vista 32bit.