Computer doesn't boot up, need help

First, i must warn you i'm not entierly computer savy, so exscuse the inherent level of N00bness here

Anyway, a friend of mine just bought a new motherboard, and we've been trying to replace his older one (which apparelty got fried for no apparent reason).

He installed the motherboard, but for some reason the computer doesn't seem to boot.

Power goes on, and all the fans and leds turn on as well(the hard-drive led stays constantly lit, not sure if that's a good thing or not), but nothing else happens, no BIOS bip, hard-drive doesn't make any sound, screen stays blank.

We tried to strip the computer down to CPU and memory only, and apparently we received some sort of feedback (the speaker made some fleeble series of noises, but apparently according to him it never worked right), which would indicate that the motherboard is alive and kicking.

After powering it the second time, we were able to smell a little burning scent, wich put a complete stop on furthere attempts to mangle with the darn thing, and it looked like it came from the CPU.

I don't know what to think. I don't have the precise specifications at the moment, but i think he as 2600 Athlon on a Asus A7V600 (KT600), sith 512mb of 2700 memory.

We double checked every step, and everything seems fine (i even added some Artic Silver II on the CPU as the older dissipation pad was comletely melted away, and we tripple checked every pin to see if it was clean before installing it on the motherboard.

I don't know what to think. Could it be that the CPU died instead and that his original motherboard was fine ?!? (He says the symphoms are similar).

He had this machine for 6 months, and according to him one day he just woke up and found the computer dead, albiet he was still powered on.

Right now i'd like to determine which peice is bomked but i'm not sure how to do this.

Can i take both memory and CPU out and power the motherboard , to see if it gives some sort of signal ?!? And if so, what type of warning should i look for ?!? (If we can determine the motherboard works for sure it could be a great already).

HDD led constantly on is not a good sign indeed. Every time this happened to me, it was RAM related. You might want to check into that. Maybe try a <A HREF="http://www.memtest86.com/memt31a.zip" target="_new">Memtest</A>.
Try also clearing the CMOS and booting again afterwards.
If you can enter BIOS, check that all your settings are Optimal for system stability rather than performance.