After I have successively and successfully installed 3 Windows XP's on the same partition (Don't ask why...), I restarted my computer again and the BIOS just froze when it loaded the video card.
Together with my uncle, we tracked down the problem and found out it's the hard drive's fault.
We tried booting without the hard drive and it worked! (No other hard drive (I have only one) but with a Ubuntu Live CD in.)
We tried the hard drive with a different data bus (It was from an identical computer) but that one didn't let my BIOS recognize the hard drive.
We also put the hard drive in another computer as the second hard drive and it DID recognize it but Windows XP kept saying it cannot install a driver and that it installed successfully. Happily, I have managed to backup some of my most important files in that other computer.


The following is a list of tests that we have run.
With the hard drive
Original data bus
Original computer
Result: BIOS freezesWITHOUT the hard drive
Original data bus
Original computer
Result: Everything works just fine!With the hard drive
ANOTHER data bus
Original computer
Result: Cannot see the hard driveWith the hard drive
Original data bus
ANOTHER computer
Result: It worked!With the hard drive
ANOTHER data bus
ANOTHER computer
Result: It worked!
During the tests, we had only two data buses and two computers. (each data bus from it's own computer)
Strange thing is that the second data bus cannot let the BIOS see the hard drive in my computer but works just fine with the other computer.
Funny thing is that the computer can't recognize any hard drive but it's original, with which it doesn't start.
I have my most important data on that hard drive and I really cannot afford to buy another decent IDE hard drive now!