You would need to start over and reinstall the original Dell Windows Vista operating sytem that originally came with your Dell computer, then use the Windows Vista Anytime Upgrade disc to upgrade. You can only activate the upgrade using the new product key that is required for the Windows Vista Anytime Upgrade. In other words, the product key affixed to your Dell computer is not usable with the Windows Vista Anytime Upgrade disc.
Windows Anytime Upgrade: frequently asked questions
http://windowshelp.microsoft.com/Windows/en-US/Help/37070041-1b90-4433-be0c-ab2855841b981033.mspx
If you only wish to reinstall the Dell Windows Vista operating system that came originally with your Dell computer, please read the User's Guide that came with your Dell computer. If you need additional assitance, please contact Dell Support:
http://support.dell.com/support/index.aspx