Debugging a New PC Build


It's been years since I built a PC and there's plenty that's changed, so since my lab needed a new PC, I decided to buy some components and out one together. I did it at home so my kids could help. Unfortunately, it hasn't had such a happy ending--I can't get it to boot. Through a series of experiments, I've decided it's either the motherboard or the CPU, but I can't decide which.

I'm using a ASUS M2N32-SLI Deluxe motherboard and a AMD Athlon 4200 CPU. I don't have anything in the motherboard right now except the CPU, the RAM, and a cheap video card. On occasion I can get the thing to come up to the point where it displays the splash screen with an invitation to push TAB to see the POST results or DEL to enter SETUP. I've even gotten it past that to the POST results (nothing interesting) once.

My theory going into today was that the CPU was overheating and shutting down, so I reseated the heatsink on it several times (cleaning it thoroughly and applying thermal compound). That hasn't seemed to do anything. The CPU heatsink never gets hot--so it's shutting down before much heat gets transfered to the sink

At this point, I believe either the CPU or the motherboard is bad, but without a spare one of one or the other, that's hard to do. Does anyone have any ideas besides RMAing them both?