Power Supply Reference Sheet posted by Thor

Here are the ATX allowances for power supply voltage spec deviation, measured at the motherboard power connector, negative test lead on the chassis frame for reference ground.

+12VDC  output  +/- 5% (11.4V to 12.6V) This rail may deviate up to +/- 10% if the supply is under peak loading.

+5VDC output    +/- 5%  (4.75V to 5.25V)
+3.3VDC output  +/- 5%  (3.14V to 3.47V)
-5VDC output +/-10%   (-4.50V to -5.50V)
-12VDC output +/- 10% (-10.80V to -13.20V)
+5VSB +/- 5%  (4.75V to 5.25V)

If measurements show that the voltage deviations are outside the ranges
specified above, then it's time to get a new power supply.