Os x mountain lion- Itunes error

Hi there, Recently i've been having an issue regarding itunes 11 whereby when i log into the itunes store i'm presented with an "upgrade to the latest version of itunes" message with no option to close this window, and this message has also stopped me from performing any online actions within itunes. Of course like most things now the online store worked fine all of the time but now apple have just decided to stop me from using the service just because "its old", I'm not asking for full support but at least the functionality of the store its ridiculous that i paid so much money for this imac in 2012 and i can barely use it in its original state. Anyhow anyone else with any knowledge on this would be very helpful as its quite an issue.

Thanks in advance