Selected Boot device not available

Hope you can help, this is driving me mad.

Halfway through installing a clean install of Vista, the install failed (i suspect due to a problem with the DVD being dirty as there is a large smudge on it)

The PC now won't boot from the hard drive, giving the error 'missing bootmgr. press ctrl+alt+del to reboot'

I thought I would be able to boot from the Vista DVD, but the DVD drive doesnt seem to be working - even when you press the eject button the drive doesnt eject either.

I then tried to copy the Vista DVD to a USB drive and boot from that, but that doesnt work either.

Some further details:-

I can't even get to an OS or command prompt.

I have read in another forum that I should take the battery out of the motherboard to try resetting the bios, which I have tried without success.

This is what happens:-

Turn PC on (without dvd in drive or pressing F12/F2) and it says :-

"bootmgr is missing, press ctrl+alt+delete to restart."

If I then put the DVD in drive (which is bootable), restart the PC and try pressing F12, i can then choose to boot from DVD. After a few seconds, it gives an error saying :-

"Selected boot device is not available"

When it does this, there is no activity from the DVD - it doesn't even sound like it's spinning. And I cannot eject the drive. The only time I can use the button on the front of the PC to eject the disk is in the first 1-2 seconds of booting.... and all that happens is it literally releases the DVD tray and then stops - I have to pull it manually out and push it in. It is almost as if when it's closed it isn't actually locking in to place.

I have even tried copying the contents of the DVD to a USB drive, and selecting to boot from the USB device. Again, I get the error:-

"Selected boot device is not available"

I am really stuck now - don't know what to do.

The only thing I have read that I havent yet tried is to change the jumper settings on the DVD, but as I have never changed these, and I have been able to boot from the DVD in the past, I don't see how this could be wrong.  The DVD Drive is IDE and the jumper setting is currently on 'Cable Select'. The DVD drive is connected to the end connector and there is nothing attached to the middle cable.

Any more ideas?! Is it possible that the BIOS doesnt have the drivers for either of these devices?

Help appreciated!