Thanks for the suggestions folks. Unfortunately, we have been installing the SATA drivers using the f6 command and that hasn't worked. We used the drivers that came with the motherboard and then downloaded the newest ones we could find on the motherboard website. In both cases we still got the blue screen of death!
We tried installing XP without the SATA drivers but the system couldn't even find the hard drive. Still, it makes a change from blue screens....
You can probably tell from this that we've tried to install XP more times than one would normally wish, this week.
Thanks for the advice. Number one son has now decided to cut his losses and live with Win 98 while he gets back to revising for his A levels - which is what he really should have been doing all along!