You are not alone in this Robert_Snyder, occasionally it does happen to me but then I realize that it is more then likely something running in the background that I forgot about. There are some anit/virus programs that will run in the background and even some defragging programs. One of the things you can do is open up Task Manager or any other program that you might have that shows the status of your resources (CPU and Memory). I leave Task Manager on and minimized with the performance tab opened. That way when ever I do a task I can give a quick glance to the Task Bar and see what going on.
If I was smart enough I could use Task Manager to see just what was using the resources at that time, I ain't.
One thing you didn't mention is whether your computer is home made or off the shelf. The reason I ask is that sometimes an HP or Dell or whatever brand could have something running in the background that you might not be aware off.
Hopefully someone will come along and response with better information.