Ah, so the known working board was the 2nd hand one referred too just above.
You tried updating the BIOS? the E6300 is one of the later intel 775 chips, was only released Q2 last year.
Have you got an older 775 to try?
Ah sorry, thought you meant the later 2.8 GHz E6300, this one here
I presume you've tried it without your usb keyboard connected, or anything else usb for that matter, just to eliminate any dodgy usb devices?