You're probably right, I just wondered if you had several applications running in the background which might have been taking resources.
The only thing I can suggest is, next time it happens, check Task Manager for running processes, and see if anything's maxing out the CPU, or check the error logs and see if there's anything untoward in there. I assume you've done the usual virus and spyware scans?
As I said before, I'm no expert, so if anyone else has any ideas, please shout.