HELP!! XP Activation Key - Can't get logged in

After seeing this message on my screen after login for months and not paying attention to it, I finally figured out what it was. Invalid Activation Key and now when I login to my system, I get that same message. But now when I click OK, I get Logging Off and it takes me back to the login screen. The OS was from a previous employer so I figured I should go get my own, so I did. Now I can't figure out how to get the activation key on this installation without blowing it out. ANY HELP WOULD BE APPRECIATED!! THANKS.

 

Choose to activate by phone, then click the button that says change product key.

Or, http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx