Vista Business product key is invalid

I bought an E6400 used from a company and they gave me all the OEM disks including Vista Business. I re-installed Vista using the OEM disk. I recall activating Windows when I did the install. Everything was going fine until I ran a repair on AVG. After the reboot a message came up saying I need to activate Windows. I try and get this:

I never entered a product key as it is an OEM disk. I have used many Dell OEM disks and have never been asked for a product key. I had a similar thing happen after uninstalling Adobe on an E6400 with Vista Business (that was at work). Is there some MS update or something that is causing Vista Business to suddenly become unactivated and the key invalid? This is concerning to me because the laptop does not have a MS OS sticker on the bottom (that was how I bought it).

Any and all help will be greatly appreciated.