As said, I installed the most recent (at the time) Debian OS from a DVD with the ISO burned onto it.

Since then, however, I've been unable to access the hard-drive through BIOS and Windows, let alone boot the system.

It's installed on a separate drive from Windows, 80GB, 7200RPM, and I know it still functions since both my Windows drive and the Debian drive spin when my computer powers on. However there's no option to boot either Debian or Windows, it simply runs Windows straight away.

I'd like to format/wipe the drive so that I can re-install Debian, hopefully without any issues.

Any help greatly appreciated.

You're saying that the BIOS no longer recognizes the drive? Minus the BIOS issues, what you described sounds normal when you install the bootloader on the same drive as the OS, which is different than the current "master" drive being used for Windows. You would need to change the boot order in the BIOS to put your 80GB drive ahead of the Windows drive in order to boot into it. However, if the BIOS doesn't even recognize the drive, then something else is going on.

Well that's the issue. Whilst I'm new to Linux systems, I'm fairly experienced with computers. The fact that my drive seems to have disappeared off the face of the earth has completely baffled me.

Can you see it in Windows?

Assuming it's Windows 7, you would go to Control Panel > Administrative Tools > Computer Management > Disk Management. You're not going to be able to assign it a drive letter since Windows won't recognize the filesystem, but it should still show up in Disk Management.

I can see the disk in Disk Management, well, I can see the disk size and its partitions, though nothing more. Is there something I can do?

Not really, but if you can see it in Windows then you must be able to see it in the BIOS. What mobo is this?

Asus M5A97 Pro. I've decided to format the drive through Windows, ready to re-install Debian when the DVD .ISO eventually burns correctly.

Thanks for your help! Glad I signed up here.

It looks like you can hit F8 when you see the ASUS logo on startup to manually select the boot device. If you run into the same problem again after re-installing, try this and see if the 80GB drive is listed as an option.

I have access to the drive again, thanks for the help.

Now to just write this ISO properly...