what have done - and HOW!?

Hello people. I have a games machine, few years old now, but always behaved perfectly, until a few mornings ago. The board is an Intel DP35DP SKT 775, with a dual-core Wolfdale cpu, about 2.8Ghz I think.
Anyway, I am in the process of building up another SKT 775 computer for a young lad, and I needed a processor for it, saw something called a Pentium D going very cheaply,so got it, but before it came, he came around with his uncles old machine, yet another SKT 775, and the processor in that was a dual, so I put that in and it worked fine. So, when the Pentium D came, I thought I should try it and give feedback for the seller, so, took my own games machine processor out, put that one in, and fired up. Nothing happened, so I assumed processor was dead and put my own one back in. Machine fired up, but couldn't find the boot disk, and then promptly shut down. I fired up again - same thing happened. Fired up 3rd time - nothing, no graphics, no disk sound, just all the fans whirring.

I wondered if the machine was shutting down because the Intel heatsink was not relaying the heat properly (yes I had put more paste on), so we ordered a Cooler Master EVO to replace that, and I contacted the seller of the Pentium D to tell him what had happened. He was very friendly and helpful and after I gave him the mobo ID, he looked it up and told me that Pentium D would not work on that board, and he explained also that the modern CPU's have sensors at the corners and will quickly shut the machine down if they sense the heatsink is not evenly in contact with the die surface.
Good, I thought, sorted. Once we get the EVO on,and it's proper processor is in situ, it will surely all start working again... well, it hasn't. None of the three graphics card I tried have worked, and the Gigabyte one that has normally been in there doesn't even spin the fan now. We have checked the voltage on the PSU, which is a replacement 600w Corsair that has only been in a week, run the Gigabyte card in another computer, where it worked - so I am forced to conclude that the E6300 Intel Processor has failed, or the Intel board has failed. But why and how? Simply because I put another SKT 775 processor in there for a few seconds? Is that possible?

thank you linnemeyerhere - I did take the battery out and then replace a bit earlier on, made no difference, and I didn't really expect it to, but it was worth a try. There is no graphics chip on this board. What do you suppose has gone wrong though? I have no idea. This has never happened before...

there's only one way the cpu will fit, made double sure it was properly aligned - can't do it wrong really. No pins on these, contact pads instead. Cleaned with Isopropyl, checked the array with eyeglass and brush. Haven't tried pulling the ram sticks out, will do that tomorrow. Have manual, don't think it covers this but will look again. No led troubleshooter lights on this home-built. Reckon this is going to be an unsolved mystery. Have bought a pulled cpu same ID, be here Tuesday. Will report back. Thanks for help, appreciated.

OK linne, hadn't thought of doing that, will do later - will say if any diff, otherwise will report back after trying 'new' processor. Thanks
Had a few mins to spare so tried it now - took all but one stick of ram out, connected audio ( you did mean the sound system I assume - no internal speaker). Anyway, nothing changed...fans spin up, one little green light on board ( must look to see what that signifies), the DVD bay will open, the power button will start and stop the machine, but otherwise nothing at all happens I have tried two monitors, 3 video cards and two psu's. No ideas left except to try a different processor. If THAT doesn't work, then it must be the board... can't believe it, but what other conclusion is there? Anyway, will post at next event.

ahead of you Linne, looking for another board (UK), but all suprisingly expensive so far considering their age - also the idea of upgrade; I know what I want, but the processor is £160. Gotta save me pennies for that!
oh - did you mean the board was offered on the Intel site? I shall look into that then - thanks.