Everybody forgets at one time or another. Especially when you are knocking out windows upgrades a dozen at a time. A week or two after you install windows 7, the user submits a help desk ticket telling you their windows is not "Genuine".
Not to worry, here is what you can do to correct the situation.
In the start menu, go to accessories, and then right click on the command prompt. select "Run as Administrator" accept the user access control warning and then type regedit into the command line and press the enter key.
Navigate to HKLM/Software/Microsoft/Windows/CurrentVersion/setup/OOBE/mediabootinstall and Change it's value to 0 (zero)
In the command prompt that you still have open,
type "slmgr -rearm" and press the enter key. (you can reset the activation period up to 4 times.)
After slmgr has shown you the dialog stating that the rearm was successful, reboot the computer.
Right click on computer and select properties, In the computer properties scroll to the bottom and click on "Change product key". Enter the activation key that is on your certificate of authenticity and click next. If activation accepts the key, you are done.
Continue on with the activation dialog and complete the activation over the phone. You can also double check your activation key as many of the digits are difficult to tell apart. I take a picture of the key with my phone and zoom it in to tell a B from an 8 and a G from a 6.
Software piracy is a crime that we have the ability to stop within our own span of control. but sometimes I forget to activate a license, enter the wrong key or make some other mistake that results in the OS going into hamstrung mode. With the instructions above, you can resolve most of the issues surrounding licensing and activation of Windows 7.
I would have loved to have known this back in my desktop support days.