installed windows 7 on dell pc running vista

The Product Code on your Dell is only valid for the OEM Windows version that was pre-installed on it at the factory.

You need to buy a new Product Key & Licence for your new Windows version.

Easiest way to get Product Key is to buy a genuine Windows 7 DVD from Amazon which is supplied with Licence & Product Key. I just looked on their website, they do still have some in stock.