Before you do anything, make sure you copied your own files (music, pictures, etc.) to a safe place (external disk, USB stick or DVD). There is no reason for you to loose your own stuff.
You could try an in-place upgrade as mentioned by Kari (although that is generally the second best solution).
In this context you have to understand that the installation of a program is something very specific to the operating system at hand. And w7 and Vista are 2 different systems - despite of what some people say.