Well if you installed Windows 7 over Vista than you'll have to reinstall Vista over top of Win 7.. assuming you have a copy of Vista... Perhaps you could provide us with a few more details in order for us to help you a little better.. like did you do a dual boot? Did you do a backup of all your personal files before you installed Windows 7? Things like that will help us all get a better idea of what exactly your situation is...
You can't install Vista over 7....
You'd likely have to do a format in that case, as I've never seen an OS that will let you install it over a new OS, even as a 'clean' install.
There should be a test before allowing someone to download and install a BETA OS.
There was a statement where I downloaded my copy from that made it pretty clear about the risks and so forth of installing a beta. Also said you should be willing to troubleshoot your own problems as there will be no support from Microsoft...
OK Stable doors and horses but learn for next time.
ALWAYS but ALWAYS keep your private data (Music, Video, Pictures, scans, emails, Data base files, address books etc etc) separate from the OS. Always create a separate partition for each OS (you don't need more than a few GB per OS).
That way if you lose the OS and have to restore it you don't lose any data.
In any case BACK UP EVERYTHING and have at least 2 different backups (on different media if possible).
I have a lot of clients photography / contracts / drawings -- so I always keep a copy OFF SITE as well .
You can never have too much backup.
Sod's law will work 100% every time --the one time you didn't backup a file is the one time you will need it.
Cheers
jimbo