Dell Dimension 2400 motherboard

Do you hear any beeping sound upon booting up the computer? do you hear any HD activity happening? or is the unit totally just halts to that?

I checked the specifications of your board and it seems ok (socket 478, DDR400)

Based from the symptoms you mentioned, I could easily assume that you have a problem with the board or the CPU but just to be sure, I suggest that you proceed on isloating the problem.

To do this, I suggest that you remove the HD, CD/DVD, floppy connections and any adapters cards (if any) that you have installed on the board (sound, lan etc.), as for the video, I believe your board has a built in video but having, you can use that instead. From there you check the major components:

Power: check Power supply fan, CPU fan, lights - since you getting lights already and I guess your fans are all ok, we could skip this part.

make sure that the CPU (+heatsink andfan) and memories are properly seated.

try swapping a known working ram module, if you have multiple ram modules, try using one module at a time, or you can remove all of them, anyway if its a memory problem, your PC should send out error beep codes to tell you about it. however, if nothing still happens, it means that your PC isn't processing anything, you'll have to isolate if its the board that you just bought or your CPU that is causing the problem, try reseting the board's BIOS through the reset jumper to make sure that it reverts back to factory on which are commoly set to automatically detect CPU voltage and ram speed, also you can try swapping another CPU or board if possible. You might end up replacing the new board or have it tested from where you bought it (you can perhaps bring the CPU and memory with you for the later part). Hope this helps

Cheers and best regards,