I got it!
I took off the BIOS battery during some hours. Then I plugged it again, so the bios returned to its normal behaviour. The hard drive continued without being recognised by the bios, so I downloaded the Windows 98 boot disk ISO, this file from the DFI web page: http://img.lanparty.tw/Upload/BIOS/CM/PB61BX0530.ZIP
and I installed the application MagicISO.
I unzipped the PB61BX0530.ZIP file, included the unzipped files inside the Windows 98 boot disk file with MagicISO and burned the result ISO with Infrarecorder.
I Rebooted the PC booting from the CD, and then it showed:a:
so I changed to the CD:d:
and typed: f741
The application asked me for the binary image to copy into the bios, so I answered:pbbzk.bin
It asked me if I want to save the current bios binary image: I anwered no
The process worked correctly, the PC rebooted automatically and it recognised the 80GB drive as master!
I realised that even the user's manual says the motherboard revision is DA+, just in the motherboard I could read "rev. D1", so I began to find the bios version for the D revision and not for the A revision, which was confussing at first.
Thanks to you people!