That version of Windows is tied to your old IBuyPower system. Per the license agreement, you cannot just take that old hard drive and put it into a new system (I'm surprised that worked without a reinstall). The fact that you have been using it for some time now without issue is a moot point. Microsoft has determined that your license is not valid and needs to be re-activated. Click on the Activate Windows Online Now option and see what happens.
More than likely, online activation will fail and you will be asked in you would like to attempt activation over the phone. If so, choose that option and attempt activation that way. If that fails, you may be prompted to contact Microsoft directly and a technician may re-activate the OS for you. However, if none of the above work, then it's on you to purchase a new Windows license.
-Wolf sends