Hi,

A lot of manufacturers tell you to attach the processor and cooling unit before inserting the MoBo into the case. That doesn't mean that you have to remove the MoBo subsequently.

Socket 775 units look like this: http://www.pctoys.com/840556058366.html

As can be seen, there are 4 mounting screws/fixtures so it is completely different from a socket 478 unit. From this, I would assume that you were not able to attach and run it.

I do hope that you carefully replaced the little insulating washers when reinserting the MoBo, typically, they are orange in colour. I would also check that there are no lose screws or wires touching the MoBo, that might cause a short.

Also check that the CPU fan is connected to the MoBo correctly, and that any sensor wires are also correctly attached.

It turns on for a couple of seconds, and the keyboard lights flash. That suggests that the BIOS has initiated. Do you get any bleeps?

Check all connections from the PSU, to ensure that they are firmly and correctly attached. Also check that memory and graphics cards are clean and firmly seated.

We really do need some more information