Do a defag to start. Try opening up the task manager and look at the processes, then hit show desktop and see what's consuming all the cpu. My guess is the explorer.exe is eating it up, why exactly could be a number of things, of which I am not qualify to say. Did you recently install something new before this emerged? Try isolating it down to the past few things you've changed about your system and go from there.

Or do a format if defrag doesn't work :/ shrugs.