Thanks to BootMaster (and marioman), I think (more like hope) that I'm closing in on the source of the problem.
For some reason, the 160 GB hard drive I installed several months ago after my first HD crash is being seen as being 137 GB by my BIOS, something which has to do with older motherboard not supporting 48 bits addressing or something like that. Boot Master reports that my main partition goes beyond those 137 GB, which I'm pretty was the case before. At the very least, XP saw a maximum of 149 GB before (which is about right from what I've read about difference in space calculation) but I have no idea if my BIOS saw it correctly before. I assume yes, but I can't be sure.
It also sucks that Boot Master can't actually fix anything unless I register it, which I'm hesitant to do considering it might not even work. At least I've managed to back up all my relevant data thanks to Knoppix, though I hope everything I burned with it is readable by XP if/when it runs again.
At any rate, unless my BIOS can see my HD correctly, I can't do much. For one, I haven't found any BIOS update for my mother board anywhere, much less on the manufacturer's website (ECS K7S5A board), but since it worked before, it could also be the IDE controller which is damaged in some way. And even if my motherboard saw the disc correctly, I still don't know if my partition would boot correctly.
Man, this is a real nightmare, and I really dearly hope I don't have to reformat and re-install everything, which takes 3 or 4 hours just for Windows alone...
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"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably." [...] The first time any man's freedom is trodden on, we're all damaged. - Captain Jean-Luc Picard
B*tch, meet reality. Reality, meet b*tch. - Me