Why does CPU usage fluctuate when nothing's running

Archived from groups: microsoft.public.windowsxp.perform_maintain (More info?)

I have a Dell 1 GHz running Windows XP Professional. I
also have Norton Anti-Virus 2004 Professional installed
and enabled and I frequently run the most recent version
of Lavasoft's Ad-Aware software to remove spyware /
malware. None the less, I noticed that my computer was
making noise when I wasn't using it.

I pressed Ctrl-Alt-Delete and noticed CPU usaged was
cycling between 0% and 99% every two to three seconds. I
clicked on the Applications tab and no programs were
listed. I then clicked on the Processes tab and saw a
long list of programs most of which were idle. Four of
them, however, indicated CPU usage:

Taskmgr.exe ranged from 00 to 01
SAVScan.exe ranged from 00 to 99
qbupdate.exe ranged from 00 to 03
System Idle Process ranged from 02 to 98

Are these processes and usages normal or have I installed
something that is slowing my computer ?

On a related issue, I have my screen saver set to display
my photographs. When it is running, images will begin to
appear on the screen, freeze for a moment, and then
finish appearing. Why is the image freezing and is there
anything I can do about it ?

Thank you for any help or advice you can give me.

Sincerely,
Michael