I would suggest taking out the RAM, modem, etc. See if the board produces any beep codes. If it does, then we'll start from there ... and, if it doesn't, I would vote for bad motherboard.
On majority board that I've seen, the CMOS will just reset back to default settings. I suppose it all depends on the BIOS. I mean, the computer will boot without a CMOS battery, so I wouldn't think it could prevent it from booting.
Generally speaking, if the board produced no beep codes when the memory was pulled, I would usually say it is a bad motherboard. In some scenarios, I suppose it could be the power supply ... but I couldn't say it would be very likely.
Sounds like your BIOS is scrambled. I gues you've tried this but if not then give it a go.. Take out the CMOS battery for atleast 10-15 mins then put it back and try again.