If possible, check the voltages on a spare power-supply plug using a voltmeter. Black is ground, yellow is +12V and red is +5V. If you don't have a voltmeter or multimeter but feel that the power supply is borderline/bad, then replace the power supply. If your computer is a pre-built system from HP, Dell, Compaq, Gateway or so forth, then tell us what brand and model it is. If it's a standard ATX system like most of us build for ourselves, then you can buy a new ATX power supply at your local CompUSA or Dabs or whatever you have.
I emphasize the benefit of buying a quality brand of unit, even before worrying about the wattage rating. Antec or Enermax are a couple you'd be likely to find, with decent quality. 350 watts is a good starting point for a recent system that you might put a high-end video card in later (Radeon 9800 series, etc).