Did you make a system image before performing the upgrade? If you did it would only be a matter of restoring that image. My guess is that you didn't so now in order to go back to Vista you would need to do a clean install. Since you need to do a clean install anyway, why don't you try installing Win 7 that way first to give it a chance? Most experienced users will tell you that upgrading is not the best way to go and is often problematic.
Make sure you do a backup of all your data as you will need that whether you go back to Vista or move forward to Win 7. All of your apps will need to be reinstalled.
In case you need it, here's a link to a tutorial
to do a clean install with Upgrade version of Win 7.