I have a three year old Compaq desktop with Windows XP. The power supply died and the computer shut down. I relaced the power supply with a new 350w unit and now the computer will not boot up past the "post up" screen. There are two lights on the front of the tower, a green one and a red one. The green one comes on but the red one doesn't. I hear the fans on the power supply running but the hard drive is not spinning.

The keyboard doesn't work. The cd drive is getting power because the light comes on and the door opens but it will not read a cd.

Any and all suggestions on what to do next would be greatly appreciated!

First ensure all physical connections are secure. If it still exhibits the same results it may be worth investing in a power supply tester. Computer stores sell them for around $15.00 US. (Could be you got a dud power supply. It happens.) Or ask where you purchased it if they will swap it out for another to verify PS integrity.

"and now the computer will not boot up past the "post up" screen. "

The fact that you're getting a POST screen signifies that the motherboard/CPU/Memory is somewhat still intact.