Mysterious boot problem

I have a problem that no amount of Googling or asking friends has helped.

When I try to boot my PC my fans come on as usual, but that's it, it doesn't boot. However I can boot from a live CD if my external HD isn't plugged in.

Then, when it is booted from the live CD, my internal HD is accessible as usual, and if I reboot I can boot from it as usual, but again, not if my external hard drive is plugged in.

Any ideas as to what the problem could be?

The BIOS is probably putting the external HD above the internal HD (and possibly the CD drive) in the boot order when it's plugged in, and so it tries to boot from the external HD first, which of course fails. Go into the BIOS and just set the internal HD to boot first.