Case,Power Supply,and Shutdown

[LJ76]

I just moved my super crap Compaq into an In-Win full size case with a 235 watt power supply(the Compaq had 110 watt!) Anyhow, I'm very new to screwing around with the computer like this and probably have no business doing it. However, almost everything went well without too many problems except for the fact that occasionally the system will spontaneously reboot now and once when I tried to shut down it just hung at the "Windows will now shut down" screen, which led to an improper shutdown that caused a lot of errors. Can anyone speculate on what might be causing this problem and how I might be able to fix it and get everything stable?
Thank you

[Maxout]

Check to make sure all connections are tight and that CPU/RAM/AGP & PCI cards are well seated. Are your fans operating? Do you have adequate case cooling?

[Toolbox]

Computers suddenly reboot on their own when the processor gets a bit too hot. What chip do you have and what heatsink and fan are mounted onto it?