Computer will not turn on

[Tech1292]

I have an older HP computer (a630n) with a Pentium 4 processor and an ASUS PTGD1-LA motherboard. I unplugged the CPU power cable (4 pin square cable) and put the 20 pin motherboard cable in and the power supply spun slowly. I then inserted the 4 pin cable and attempted to turn the computer on. No luck. The computer seems to only get power when the 4 pin cable for the CPU is unplugged. What could possibly be wrong?

Note:
I have no add on cards installed.

 

[te100]

power supply dying maybe