Ok I don't really know what the problem is so I've put this in General Hardware...

Basically my computer was crashing a lot, which I figured was down to my hard drive, and since I needed a new hard drive anyway I bought a newer better one. However, when I replaced it and everything, suddenly the computer won't start up at all, not even a BIOS screen - nothing on the monitor. I tried putting the old hard drive in, same thing. My friend suggested switching it on with no hard drive, again same thing. He suggested it was a RAM problem.

I tried taking the RAM out and putting it back in, I tried it in the other slot, nothing.

When I try it with no RAM at all, the computer just switches on (the CPU fan starts up so I assume it's not a power fault) but of course nothing on the screen. I have to switch the power supply itself off, not the off switch on the front of the case.

When I put RAM in, the CPU fan start up, nothing on the monitor, I hear two quick beeps followed by 8 longer beeps, and I can switch it off with the button on the front.

My motherboard manual doesn't say what 8 beeps means, it's a K7S5A if that makes sense.

The monitor makes a response when the computer comes on, so it's connected and everything, but it's obviously not getting a picture sent to it, i.e the BIOS and everything isn't starting up.

I was thinking I should get some new RAM to replace the old RAM and maybe that would fix it, am I right?

Oh my... after a bit more research and googling, I've fixed the problem myself. For future reference here's what was wrong:

The motherboard is a K7S5A. I was getting 8 beeps after the initial power-up beep, which it turns out means there's a problem with the graphics card. For a minute I was worried that the graphics card was broken and I'd need a new one, because it appeared to be seated correctly. However, it turned out it needed to be pushed in further than it seemed. I pushed it right in there, and hey presto, my computer's working.

Good news. :)

"If it ain't broke, upgrade anyway."