That's the problem with computers.. it could be anything. If my computer was freezing randomly with a dual core 2.6 processor and 2gb ram, the first thing I would have turned to is the hdd... Expecially with XP on it, there is no reason for freezing.
Test the drive, run some scans. If you did in fact replace everything else, then my theory at first would have been right
it's the hdd...
What sort of processes are you running?