>I needed to install a new HD with its corresponding OS (xp) in an old Pc (500 mhz, 128 ram, bad CD-ROM), but due to the slowness of the Pc, I prefered to install the OS by using a newer PC. Everything went fine but when I installed the HD in the old pc, XP wouldn't start. My questions are: is it feasible what I tried to do? Could it be that the OS is not working because the BIOS is not recognizing properly the HD? What else could be causing this problem?
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>It seems like the Auto-detect function of the BIOS is working fine but I am not really sure...

When you install XP it installs a lot of hardware-specific drivers. If you then put that hard drive into a different computer with different motherboard, CPU etc. there is an excellent chance it won't start at all. I've seen a few cases of this that actually worked, much to my amazement. It's a testament to the resilience and self-repair capability of some Windows OSs, but you certainly can't rely on it to work in all, or even most cases.

There is another problem with XP - if you activate it on the new PC, then put the HD in the old PC it will alomost certainly fail the anti-piracy test and need to be re-activated. Hopefully you didn't activate XP before putting the drive in the old PC.

Install the OS in the computer it's intended to run in permanently. If the sluggish CD-ROM is the problem, temporarily install a better one just for the installation.