The 2.0V PIII-600 is an incredible power sucker... perhaps way over the mobo regulators' limts.
To check if the cpu is ok, try underclocking it to see if your system fires up or not.
Comparison: PIII-450 vs. 600
Nominal/Typical Power: 17W vs. 23W
Maximum Power Draw: 25.3W vs. 34.5W
(info taken from
www.sandpile.org)