Is it the processor or the hard drive?

My machine has been locking up very frequently? I think it is hardware. It has locked up when the screen saver has come on and will not let me work with it again. It has locked up on reboot on the first blue scree, while checking the hard disk onthe first blue screen, while downloading SP6a and while trying to install it. The only way to fix the problem is to unplug the box, the power won’t even reset. I have changed out the memory and the same problems. HELP.