IF it is the PSU then you don't have to fit exactly the same one. If in doubt go for a higher rated one than is already installed. It's worth checking what main connector is on your motherboard (20 pin or 24 pin), although the vast majority of PSU's aree in a configuration called "20+4". i.e. they will adapt to either of the motherboard configurations.
It's worth checking the windows Event Viewer to see if you can glean any information from it as to the cause of the crash. Assuming that the PC is in regular use, a couple of weeks between crashes would indicate that it may not be a hardware failure.
To access Event Viewer:
Start Menu - Run
You are looking for Critical failures (red circle with a white cross in them)