Sounds like the power supply or something loading the power supply.  Before you buy a new power supply, you can try disconnecting and/or removing any devices you don't absolutely need.  i.e network card, modem, CD/DVD drive, external devices.  I had my wireless Ethernet device shorting out my USB port on one system.  But the power supply is a common failure item and the most likely culprit.  It's always good to have a spare one. 

Irene-