If it wasn't worth doing I assure you that I wouldn't have spent my time typing it.
The reason you do it is because when you fit a new HSF on a CPU when the board is already mounted in the case, the board flexes, or bends. If it bent enough, it will dislodge components just enough so that they don't function. Usually it is the RAM or the VGA that is affected. It is a very common problem.
Don't bother disassembling your whole machine just yet. Reseat your components and try again. It's also unlikely that any of the plugs have come off the front panel header, but I suppose it doesn't hurt to look, seeing as how you are there anyways. Don't be surprised if they're all plugged in though.
This doesn't have anything to do with the BIOS or anything else. You haven't touched anything to affect that. Installing an HSF or a CDROM doesn't screw up a BIOS or melt anything down. Worst case scenario, you have an Athlon XP and you chipped the die.
You might also want to make sure your PSU is still good. It's easy to check by plugging another one in. It will take a couple of minutes and rule out the possibility. Only do that if reseating the cards doesn't work.
As long as you didn't apply too much pressure to the clips when you installed the HSF, the board should be ok. I'm also assuming you didn't use a screwdriver to install the HSF and slip and hit the board. As well, I'm assuming that you didn't knock a capacitor when you installed it. Some board designs see the capacitors installed precariously close to the CPU, making HSF installations a challenge to say the least. To the uninitiated it would be entirely possible to actually damage the board. It's stupid, I know, but what can you do?
