Windows

I hate to ask the obvious btu what is running?

Do the CTRL-ALT-DEL thing and look at the Processes. Click on the CPU button twice and you'll see what's using it most.

Which processes are taking up cpu time? Which apps are running? Some apps can crank a lot while still not using lots of memory. Check the high cpu-use services for more info at the following website:
http://www.answersthatwork.com/Tasklist_pages/tasklist.htm
Run a full virus scan and malware scan with up-to-date definition sets.

I have experienced the same problem that you are now having. I used the task manager as mentioned, shut down all apps and pin pointed the process that was using the cpu. Used the search to find the program, I my case it was the executable for my computers touchpad software; it was apparently not compatible with Logitech drivers for usb trackball. Uninstalled touchpad software and Logitech drivers are now controlling touchpad also and everything is happy! Of course you need to use common sense and not uninstall any critical components! Hope this helps.