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 HDD's fault.
We tried booting without the HDD and it worked! (No other HDD (I have only one) but with a Ubuntu Live CD in.)
We tried the HDD with a different data bus (It was from an identical computer) but that one didn't let my BIOS recognize the HDD.
We also put the HDD in another computer as the second HDD 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.

1. With the HDD
   Original data bus
   Original computer
   Result: BIOS freezes

2. WITHOUT the HDD
   Original data bus
   Original computer
   Result: Everything works just fine!

3. With the HDD
   ANOTHER data bus
   Original computer
   Result: Cannot see the HDD

4. With the HDD
   Original data bus
   ANOTHER computer
   Result: It worked!

5. With the HDD
   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 HDD in my computer but works just fine with the other computer.

I beg you to help me! I have my most important data on that HDD and I really cannot afford to buy another decent IDE HDD now!