Power Loss

Hi all.
Got a snag with my old computer which somebody out there might know how to fix.
Just upgraded to an ATHLON 2.2 chip, so swapped my hard drive, cd writer & DVD rom drive from old case into new one.
Put a 30gb drive, an older cd writer and CD rom drive into the old Athlon 800Mhz computer with 512mb Ram. Switched on and nothing happened - no noises, no power pack fan running - completely dead. Assumed tha it might be the cable connections, power lead, electrical socket, ram placed correctly on motherboard - checked it all with nothing apparant. Even bought new power pack thinking that it must have died by shear co-incidence when I swapped the bits around - this has had no affect, so it's not the power pack. Unplugging and reconnecting the ATX power connector from the board gives a quick burst of life to the CPU fan then nothing again.
Cleared the BIOS settings with the motherboard jumper - still nothing.
Next step is try a new motherboard on the assumption that I must have killed the old one - but thought I'd pick your brains first - don't really want to commit to another £47 to find it's not the cause.
Can anyone help? Might this be something you've had? Any ideas?
Thanx
Desperate Phil of UK