Pepperh is right. It may just be something doing some sort of scan, whether it be virus checking or spyware checking, maybe even a defrag is running.
If you are unable to identify (if it appears not to be a virus scanning program or such) what is taking up the CPU usage at the moment when you bring up the Task Manager (CTRL-ALT-DEL) as HAL suggested, then try one of these programs to get more info on on the processor in question you find.
What's Running 2.1.1
http://www.whatsrunning.net/whatsrunning/main.aspx
Quick Access InfoBar
http://www.processlibrary.com/quickaccess/
Win Tasks Pro
http://www.liutilities.com/
Let me know if you need help with any of these. With HAL and pepperh's suggestions, you may not even come this far.
Let us know what it was too.