Running processes

I am running XP Pro w/ a 233 mhz processor. I am constantly getting “not responding” errors and when I check, my CPU usage is at 100%. I know I need to upgrade, but cannot afford to do so right now. Can someone tell me how and what processes I can safely stop without interfering with normal operations. I seem to have an awful lot of processes running all the time.
I have plenty of harddrive space and plenty of Ram.
Thanks very much in advance for your help!!
Jeff