So i just got fallout 1, 2, and tactics and after playing for a reasonably short time my PC shuts off. This only happens with REALLY old games or some emulators. I've already checked my power source and it is fine.
PC specs.
Processor: AMD Athelon 64 Processor 3800+ 2.40 GHz
RAM: 1 GB
32-bit system
Graphics Card: GeForce 6100 nForce 405

Maybe while running these old applications, the system somehow isn't aware of your user input (mouse/keyboard) and thinks it is idle and then goes into suspend mode or shuts down. Try turning off all power saving and screensaver options and check if it still happens.

One thing that most old games will have in common is that they'll peg a single cpu core at 100%. It could be an issue with your cpu cooling, if it can't get heat away from a small region of the cpu fast enough.

Check your ddraw.ini for the ProcessorIdle line, change it to 1 and uncomment it if it's commented out. If that magically makes the game work, you need a new cpu fan or to remove the dust from the current one.