CPU USage on Win 2000

HI friends:

I have 8i installed on Win2000.
The user figures out today that the cpu usage is in big swings when oracle is up and running. every 5 seconds the cpu usage goes from 1% to 2% to 100% and same happens..
Checked on the system if any background processes are doing anything, but only 8 entries are esisting in v$session and v$process (6 for bg process, 1 for the remote login)..
Total SGA allocated is 120 mb and system is of 320mb..

I repeated the same thing on another machine and that is absolutely normal...

Any direction or clues is appreciated..

Thx