Did they use your power supply or their own? And if their own which one did they use?
What would happen if your PSU can't cope with the load? Here are a few possible issues:
bad 3D performance
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down.
The psu is one of the MOST important components in your machine. If you don't get a good brand name and adequate power across the board then you're going to have these kinds of problems.
The other thing I can think of is temperature inside the case. open the side and see if it last longer - as long as it's fairly cool inside your room of course. Try monitoring your temperatures of the gpu and cpu and chipset in general. Heat will cause freezing and shutdowns also. The slow boot times could be an after effect of windows not shutting down properly. That, and your components might take a while to cool off if it's a heat problem.