Computer error on start-up

Hi all,

I have finally finished building my new PC, it's been going on since January! I started it up this morning, my specs are as follows

xfx s775 nforce 630i MOBO
Intel Core 2 Duo E8400
800W PSU
Nvidia Geforce 9800GT 1GB
4GB PC2 6400 RAM

I'm using the IDE hard drive from my old system at the moment, as I can't afford to buy a new OS. The system started fine this morning, booted as normal and loaded XP. I had to re-activate Windows, which I had expected, and that worked fine. Then I had to go to work, and when I got back it all went wrong. I have tried to boot again, but just after it gets past the BIOS screen, the computer comes up with the error "Disc boot failure, insert system disc and press enter". The HDD is definitely being recognised in the BIOS, as it's IDE and the BIOS is showing a Hitachi IDE drive.

I'm puzzled because I thought this error only applied to discs without any OS loaded.

The wierdest thing is that I've had to put the hard drive back into my old system to use the internet, and it's working fine! (it says the hard drive specs have changed since activation, funnily enough)

Any help would be much appreciated

Fiona