While the other replies are accurate they don't really answer your question do they?
www.blkviper.com has a whole bunch of info on system processes, what they do, and how to configure them. If your system is still chugging away while you seem to be doing nothing then it's probably the indexing or boot optimizer running. You really don't need indexing if you are a stand alone home user. So turn that off. The boot optimizer function will run about every 3 days of CPU up time and only when it notes your system is idle for an extended period of time. System restore will also run for a short burst when your OS is idle. System restore is really quite a useless piece of programming so like the indexing turn it off. Good luck.