I read your post again and the issue is different than at the link I provided. As I understand it you can use LR with specific changes to your Preferences & Settings and they "stick" for a few months. At some point LR asks you to renter your serial number and appears to reset its preferences file to the defaults settings.
The most likely cause of this issue is logging into Windows after a system Shut Down with a different User Account name. I just tried it with my Windows 7 Guest account login and the first thing I see when launching LR is the Adobe registration and serial number entry screen. Make sure to use the same Windows login account when working in LR and this issue should stop.