How Do I Install Windows Vista When I Already Have Windows 7?
ok so i have windows 7 but it stopped working for me . so i want to install vista instead . i already have the vita dvd with me . do i just insert it in and install . or what do i have to do exactly

Just insert the Vista DVD and reboot the computer, and if it asks, boot to the DVD, and it SHOULD reinstall automatically. Just follow the onscreen prompts.
First, you have to boot from the DVD, just put it in the drive then reboot. When the Vista installer launches, it should give you an option to partition your drive. If you want to keep Windows 7, use the partitioning tool. If you don’t, just skip it and it will over write win7. You’ll have to put in your license key.