And another piece of info. To try and work out if this is a windows problem i decided to try and boot from the Dell Windows OS DVD. It didn't work, instead booted straight from the HDD.
This suggests that either the DVD drive is at fault, or there's a motherboard or BIOS issue.
I'll try a CMOS BIOS reset this evening to see if that works, otherwise I'll be buying another DVD drive tomorrow to see what that does.
If that doesn't work, I have no idea what can fix it.