I have an Intel Socket775 DP43BF that I recently started building into a system for my mother. To get in to the BIOS on it the first time it booted using my USB keyboard, I basically had to press F2 about 30 times from the instant it beeped until the Intel splash screen was displayed.
If the motherboard has a CMOS reset jumper, you might try following the instructions in the manual to reset it to defaults to see if that makes a difference. You might also try another USB port.
The PS2 keyboard will likely work better, though.