Product Update Available

I purchased Norton IS (2009, 2 year subscription, 3-PC) from a legitimate 3rd-party supplier about 15 months ago and recently I have been getting message pop-ups asking me if I want to update to Norton IS (2011).

 

When I try to do this, it installs OK but in the final stages, it rejects my current product key and I have to re-install the old version.

 

Anybody know why this is happening?

 

Current Norton IS version: 17.8.0.5

 

thanks