techman, I am afraid it might not be possible, at least not in any ortodox way.
If you do a clean install you shoudl end up with a windows.old folder, which contains all your files previously in Vista, with the respective locaitons. You then have to move them to the final place, but also reinstall the applications.
I beleive you might be able to use Windows Easy Transfer tool, although it is meant for migrating XP to Windows 7, but I see no reason for not working with Vista.
In any case, please backup your personal files before attempting anything. There are chances yuo have problems in the process and have to reformat the drive, when you will regret if you didn't backup first.
This MS tutorial
should also help