It could be a bad case of resource allocation.
Go to the task manager by using the keyboard, then leave the computer to sit for 10 minutes. Come back and record the approximate CPU load. Continuously move the mouse for 30 seconds, then see what the CPU load is. If it's the same, then it could be a larger underlying issue. If it jumps a significant amount, see what the program is that is causing such usage, and see what you can do about that program.