A computer I recently upgraded with a new motherboard and processor and memory and power supply(basically replaced everything) turns on for a few seconds and then turns off again when I hit the power switch, nothing even gets displayed on the monitor yet either. What could possibly cause this?

Did you apply thermal compound between the CPU and heat sink? Is the fan for the CPU running?

the fan for the cpu is running, however i don't think i did put thermal compound between the cpu and heatsink.

That could cause overheating which would shut the computer down quickly.

I'm sorry, I guess I did put thermal compound on the cpu, I should've checked before posting, I have a bad memory. What else could cause it? I have a abit kv8 pro motherboard with a 1.8GHz athlon 64 processor and 512MB DDR RAM, and an nVidia TNT2 video card. There are no beeps at all and the monitor displays nothing before the computer shuts down automatically not long after being turned on.

Did you reformat the hdd and reinstall the OS after you installed the new motherboard?

The first thing that comes to mind with that description is overheating. Is the fan on the CPU heat sink working, is there any change if you leave the case open?

I wouldn't have been able to do anything with the hard drive because it won't turn on. The fan works and it does the same thing with the case open. If it's overheating, would it have damaged the cpu or motherboard?

Have you tried assembling the motherboard, CPU, RAM, and PSU outside the case to determine whether you are shorting to the case?

When you upgraded the PSU did you purchase a 24 pin type with the p4 connector, and are both connections made on the motherboard?

Have you taken a voltmeter to the PSU to determine that all of the rail voltages are normal?

Have you gone back over all of your connections?

I took the motherboard out and noticed that there were what looks like burn marks on the back of a small portion of the board. I'm pretty sure it shorted to the case. Could shorting to the case have possibly damaged some of the other components? Could I just replace the motherboard and everything would be okay?

Yes, a short to the motherboard could damage it and the CPU. Did you try turning on out side of the case?

If the motherboard works outside of the case use the standoffs that came with the motherboard and reinstall it and you should be ok.