100% processor-time

Hi,
I seem to have a problem on a PC with W2K sp2 installed.
The processor indicates it?s working 100 % all the time.
I?ve checked all processes etc. but can?t seem to find a cause for this. Monitoring shows that the cache is always working for 100%.
Are there any of you that know this problem and have a solution?
TIA
George

The machine is a Compaq Deskpro EXD/P733 . And yes, the latest BIOS is installed.