Simple solution here, Freddie... DON'T reinstall Windows just yet, and panic not about Viri until you've tried a manual BIOS tweak...
The boot disk is decided at the start by the 'bios'... that horrible black screen with white writing that comes up before Windows.
Simple solution... most machines have a 'change settings' button during the bios boot. You need to hit that button almost as soon as the machine boots.
What has happened is either your internal battery which holds clock/bios settings has died, or simply your bios is corrupt, and the machine doesn't know which disk to boot from.
When you get to the 'change settings' screen, you need to let the machine know that your hard drive exists... the 'auto' setting should work.
Messing with BIOS is scary... and I'm halfway through a second bottle of Chilean red (which is taking it's toll, nicely

) so some kind soul on here should be able to guide you...
But rest assured, almost EVERY time I've encountered the problem you describe, it's just a bad bios.
All you need to do is let the computer know (once) that the hard drive exists, and it should work again!