I've been dealing with this issue for awhile. I have an ASUS M2N32 SLI Deluxe motherboard and with it I was given an AMD Athlon X2 4400+. According to the CPU compatibility, it is a supported CPU.
Click on CPU Support tab.
ASUS - M2N32-SLI Deluxe/Wireless Edition
The problem I thought might be that the guy gave me the CPU in a box with a bunch of other random computer stuff instead of leaving it in the mobo. Lots of bent pins and some thermal paste on a couple of them. After attempting to clean it as best as I could and bending the pins back into position I plugged it in and nothing. No beep codes (speaker attached), no POST. Took out the RAM, Starts with the RAM error beep code. Try different RAM, different video cards, different slots for ram and video cards, reseating the processor. Nothing.
At first I think the motherboard is dead, but to check, I order a cheap Athlon X2 4400+ off ebay (says tested working). Of course the guy didn't pack it well and several of the outer pins are bent. No problem, I've done this before. I get it to fit in the socket but then I see the same issue no matter what I do.
Then I find a forum post that says the BIOS may not support the processor. So I buy the cheapest AMD CPU I can find that supports the lowest number BIOS firmware, just to be sure. Bought an AMD Athlon 64 3200+. Plug it in (no bent pins finally). After screwing around and trying to get it to work turns out one of my sticks of RAM was bad and it POSTs. Awesome. But it says the BIOS version is 1704 and the 4400+ only needs 1402... I go ahead and update it to 5002 anyway. BIOS update works fine. Swap CPUs to the tested working 4400+. Nothing again. More troubleshooting yielded nothing. Swapped with the original 4400+. Nothing. Troubleshooting yielded no results. Just powers on, no beeps, no POST. Swapped back to the 3200+. POSTs fine.
PSU is an Antec 450W. The 4 pin CPU is plugged in.
It supports the CPU, so did I get a tested working CPU that's not so working...? Is there something else you could think of? While I can use the 3200+, with this nice motherboard which I now have working, a dual core would be nice, especially since I already have 2 of them...