hardware troubleshooting procedure?

I have an XP Pro workstation that is crashing and freezing at random times. after updating drivers and checking through most obvious things I did a clean install of windows on a clean drive. it has just frozen again, so – it must be a hardware issue, right?
where do I start – power supply, memory video card?
whats the best procedure to track down the culprit?
I ran a memtest on it but there’s 4gb ram, so it takes all night and then was stopped at 75%.. (?)
should I pull the ram sticks down to jsut one and try it from there? disconnect bits and pieces to see what happens or?
how do I tell if its the power supply?

any help gratefully received, thanks, paul