How have yu tested the PSU?
My basic troubleshooting procedure:
First, the new build checklist:
http://www.tomshardware.com/forum/261145-31-build-post-checklist
Yes, I know it is not a new build, but you never know. This is an application of my DSSF (Do Simple Sh!t First) principle.
Remove the CMOS battery and wait, oh, 15 minutes. This works just often enough to be worth doing.
Next steps require a system speaker. That's the little thingie that gives a short beep when your system boots.
Disconnect
everything from the motherboard except the CPU and HSF, the two power cables going to the motherboard, and case power switch. Boot. You should hear a series of long single beeps indicating missing memory. Silence here indicates, in probable order, a bad PSU, motherboard, or CPU - or a bad installation where something is shorting and shutting down the PSU.
If you get the long beeps, add a stick of RAM. Boot. The beep pattern should change to one long and two or three short beeps. Silence indicates that the RAM is shorting out the PSU (very rare). Long single beeps indicates that the BIOS does not recognize the presence of the RAM.
If you get the one long and two or three short beeps, test the rest of the RAM. If good, install the video card and any needed power cables and plug in the monitor. If the video card is good, the system should successfully POST (one short beep, usually) and you will see the boot screen and messages.
Note - an inadequate PSU will cause a failure here or any step later.
Note - you do not need drives or a keyboard to successfully POST (generally a single short beep).
If you successfully POST, start plugging in the rest of the components, one at a time.