Your Dell machine was probably originally shipped with Vista installed - for what version and edition of Windows is the COA sticker vaild?
The current Key is a KMS Volume license for Windows 7 Pro - which is only valid if you are a member of an organistaion with its own KMS server. Are you a member of such an organistaon?
If not - from where did you acquire this copy of Windows 7 Pro? - does the installed Key agree with the Key shown on your packaging?