Aiglon,
If the error message is to do with the File Allocation Table, it sounds as if it is the disk, not the MOBO that's the problem.
You should be able to query the BIOS to see if it "sees" the HD at boot - F10 at boot on every Compaq I've ever used.
If the HD is seen in the BIOS but can't load the O/S then it will be a duff HD - or possibly just a corrupted FAT / MBR (master boot record). If either of the latter, then you may be able to re-partition, re-format and re-install an O/S.
If you are going to get a new HD, be careful of the size you go for - depending on the age of the PC the BIOS may not recognise large ( > 60 GB) disks. I have a Compaq Deskpro PIII 600 that's about 4-5 years old and it won't take a disk bigger than 60 GB.
You can always look up Compaq / HP website - you have to search, but almost anything you need to know about your PC can be found (eventually).
If you live anywhere near Twickenham PM me!
SD