I would appreciate any advice...The first problem that appeared with my pc was my modem quit working. A few days later my mouse quit working. While I was in control panel TRYING to add another mouse, my LCD screen went blank. I turned the LCD off and on again--the LCD brand logo appeared, then nothing. But my pc was still on. Is this a motherboard or power supply problem? Thanks for the help! There are two ways of going about testing the power supply.. You can swap in a supply of equal or greater wattage and if the problems go away, then you've found the culprit.  If problems PERSIST, then the board is likely the problem.

The other way is finding a voltmeter and testing the psu DIRECTLY.  If the values measured fall within +/- 5% of the voltages specified then you know the psu is the culprit..

See here for the break down of the voltage pinout of the psu..

http://en.wikipedia.org/wiki/ATX


Don't bother with the negative voltages

edit: make sure the computer is on, if you decide to TEST the voltages with a voltmeterI'm almost positive that it's not the PSU.  I would replace the motherboard with a known good one first.  Power supplies will FREEZE your computer if it's going out.