Adobe Acrobat Pro xi Serial Problems

Hi,

I installed Adobe Acrobat Pro xi with no problems, first time it never even asked for the serial number. A few days later, Adobe opened a window that said to Validate my serial number. I clicked Validate, but then it claims my computer was not connected to the internet. Clearly, I am connected. About two times it has taken me to the next step of entering the serial number, but then says my code was invalid. I would have to keep entering it in, and eventually it accepted it. I have uninstalled, reinstalled again and again. The last time, it worked perfectly...for about four days. Day four: the window for the serial number validation popped up again. I just would like to fix the problem. I don't understand what I am doing wrong and do not want to continue to uninstall and reinstall the product.

Please help.

Thank you,

Kristina