Well, I dont't need help anymore.
Today, I found out that my SATA2 disk was connected to a SATA1 jack. My natural reaction was to plug it into a SATA2 jack instead, and when I booted, everything was OK (except I had to install SATA2 driver).
Then, I ran bootpart again, finding out that my linux drive was now #10. So, I updated bootsect.lnx just fo fun (I didn't think it would work).
About half an hour later (I rebooted several times, but I didn't try to boot Linux), I tried to load Linux just for fun. Imagine my amazement when it actually worked (GRUB started)! Blinded by joy, I started playing around in the GRUB menu, and selected the "windows" alternative. It showed just "NTLDR not found". I though there was nothing strange about that, after all, there were 4 "windows" alternatives (just "windows" and "windows1" to 3) and (probably) just one of them was correct; and restarted the computer.
However, after restart, I got a "NTLDR not found" (no boot manager loaded at all). I started the windows xp recovery console (from the install CD), and tried both FIXMBR, FIXBOOT and BOOTCFG. They all said that everything worked (except FIXMBR that told I had a bad MBR even after "fixing" it), but I still got "NTLDR not found".
So, the only solution I could found was to forget Windows and reinstall Mandriva (this time on MBR).
And, as of yet, I like it that way.