I have this weird problem. When I try to boot my computer, there may happen a variety of things:

1) it may either hang during POST
2) sometimes it passes POST but hangs before starting the bootloader (before lilo runs), other times it hangs when the OS should start booting.
3) it doesn't boot at all and makes beep noises (these are given a meaning in the manual of my computer, but I couldn't recreate the scenario to determine the amount of beeps since I noticed that).
4) It boots until lilo but my keyboard won't work so I can't select the OS I want to boot
5) It boots all the way and works more or less fine, but:
-at semi-random times the fans go real loud for like 1 second
-sometimes the screen (lcd) gets a bit flickery/blurry (when the cpu is used, and the fan gets real loud again
-at these moments when the fan gets really loud my keyboard sometimes won't work (also at some other random moments)

I downloaded this tool called Speedfan and CPU/disk temperatures are within normal range I think (50-60degrees celcius).

Anyone have a clue? (I did not touch the hardware and neither did I install any new devices).