I think you have already narrowed it down if freezing ever occurs in the early boot stage BEFORE windows starts to load.
My best bet would be the RAM which is relatively easy to check by substitution.
try removing and replacing first to rule out dirty contacts.
If you have more than one piece try just one at a time.
If you have only one RAM card try and borrow its exact equivalent to pop into the same slot.
To rule out windows corruption try booting from a startup floppy or even a bootable CD.
The PSU and CPU haven't had time to overheat and seldom mess about. Their problems once evident stay stuck!