[Hampshire] Problems building new PC

Help! I'm putting together a PC for a friend and it won't work.

It was a bare bones case+mb to which I added RAM, CPU, HDD - sim,ple,
I told him, I've done this hundreds of times. Spoke too soon...

At first it would power up, say the RAM was OK, beep and display the
'Press Del to enter setup' message (monitor connected to the on board
graphics via a VGA connector)

Trouble was, it wouldn't listen to the keyboard (I tried two
keyboards, PS/2 and USB) so in a fit of desperation I reset the RTC
RAM (remove battery, change jumper position, replace jumper etc).

After that it didn't even display ANYTHING on the screen, no beep,
nothing except the fruitless whirring of fans!

I really wish I hadn't done that now, but surely that shouldn't stop
it altogether? The M/b manual implied it should still be possible to
enter the BIOS setup program afterwards.

Any clues anyone?

Aaargh!

TIA, Peter