help with cpu running at 50% all the time

[gwb21471]

I have a laptop running windows xp sp3 and after it starts up it will run between 50% and 70%. The program doing it is called CSRSS.EXE. I tried to stop it and i can't because windows tell me it can't be stop. I ran my antivirus and malware program and they can't find anything on my computer. Does anybody know how to stop it from running and using up all my cpu. My poor fan keeps running on high all the time. Funny thing is it didn't do this until i installed SP3 from microsoft. It won't start using high cpu until 5 minutes goes by then the cpu fan slow gets going faster. Hoping for someone that ran into this before and know how to stop it, Thanks for the help.