I have a Pavilion 501a PC which had 1 piece of 128mb PC100 SDRAM installed.

I removed the existing 128 and installed one piece of 256mb PC100 RAM but
now the PC will not boot . I know that the 256mb is not faulty as it works
in another PC.

Replacing the original 128mb piece doesn't help as it won't boot then

When it tries to boot there are no POST beeps, the CPU fan does not spin,
but the hard drive spins up. When I power it down by holding in the power
button on the front, the CPU fan spins momentarily as the power shuts off.

The motherboard is labelled Lomita.

Can anyone help me with a way to fix this?