3 PC AVG IS reverts to trial version (on one PC only)

I have a three PC AVG Internet Security license and serial number and that serial number is entered on three computers only, all of which are Windows 7. Two laptops and a desktop. On one of my laptops, an older model that originally had Vista, AVG Internet Security is constantly reverting to a 15 day trial. I can re-enter my serial number and I am good to go. ...for maybe an hour. A day if I get lucky. I contacted AVG Support and the response I keep getting is "Your serial number appears to be installed on only two PC's." and tell me how to enter it on the third. I have done that. Many times. It just doesn't stick on that computer. The one time I messaged them and got an email back while the license was active, I get a message, "Your serial number is already activated on three PC's. If you try to activate it on a fourth, it will go back to a trial version." But I am not installing AVG on a fourth nor am I trying to activate it on a fourth machine. I Googled the problem but all I got was how to revert AVG to a trial version. There wasn't any results dealing with AVG doing so on its own. I am wondering if it is something other than the AVG License Server that is causing my AVG to revert to the trial version. Like something nasty on my PC. Customer Support isn't helping with their generic responses every time I email them about this problem.

Have you tried uninstalling AVG on that PC and then reinstalling it?