I have an older generic PC - about 3-4 years old, 30 GB HD, running Windows XP that I'm trying to revive as a 2nd computer for my family.   I took it into a repair place and they determined that the PC, which at that point wouldn't even boot up, needed a NEW power supply.  They put one in and it booted just fine.   But now that I'm home I realize it then just shuts itself down after a couple of minutes... no messages, no warning..  the screen just GOES black and it shuts down.
   I don't want to sink lots more money into this older machine but if it's an easy and reasonable fix, I'd LOVE to know.  Any ideas?
   Thanks.sounds like it is overheating. open the computer up and turn it on (just dont touch anything  ). make sure ALL the fans are working. you should have one fan on your CPU, at least one case fan  on the back (a case fan is any fan that is attached to the computer case), and , if you have a GOOD graphics card, one on that. you may have more than one case fan depending on your case, just be sure to CHECK that they are ALL working.And while the case is open, blow a fan directly in there (not a hair dryer  ) and see if the added ventilation helps.

Any other details about the machine?