Ok. Here's the Deal. The reason your OS is only seeing 503MB of a 12GIG IDE Quantum Bigfoot is because that motherboard the FIC PT-2006
has a very old BIOS that doesn't support the newest (LBA or ECHS). Which allows your OS to properly recognize the correct size of your harddrive. When this BIOS was made there weren't HDD's larger than 504MB. Back in the old days... LOL Your CPU the Pentium 166MHz is a SOCKET 7 CPU. That really doesn't matter here.
I did a little searching and came up with this page from FIC website.
The only thing you could do to fix this problem would be to FLASH the BIOS with a newer updated BIOS that has LBA/ECHS support. Unfortunately from what I saw, that MOBO doesn't have a BIOS update. There are a bunch of utilities that they have but i'm not sure if they will help you.
LOL - I'd find a sledgehammer and have some fun... IMO