If you think you have the full install media, then you can boot it up and reinstall. Just follow the prompts. It is pretty easy.
Probably wise not to format the partition you are aiming at. That way the contents will be saved into Windows.old folder ( If you have space)
You will need the esd folder if you want to try the upgrade to 8 palaver again.
To avoid formatting it by accident Do not click Drive options (advanced ) in step10.
Just point at the partition you want Vista to be on.
Clean Install with a Full Version of Vista