Any idea on what to do if you have an old computer with windows xp that has a high cpu usage?

It doesn't seem like it's a virus. It's low but once you have to load anything, the cpu goes up very high quick. Even using those registry cleaners to clean the computer is hard because the computer will shut down because the cpu is too high. I don't know what I can get rid of in the processes tab either under the task manager. I'm worried I'll just mess up the computer more.