The problem is your site template loading 2 times jQuery library (and old version!)
If you are on a Joomla 2.5, install the plugin jQuery Easy for j2.5, enable it, and select load jQUery (only) from the plugin in frontend, in version 1.8
If you are on Joomla 3 (i think you are...), change your site template, this one is not coded for Joomla 3! (as no extension should ever load jQuery library, Joomla already does!).
But, it seems an Artisteer template... and Artisteer creates only bad templates for Joomla 3 (really, all users using this have problems...)
It's better to use
Template Creator
(really Joomla 3 coded!)
This is just my opinion...
Of course, maybe your Artisteer is not updated, and the developers has changed their coding to be a real good code for Joomla3 (with the lastest version), but the last time i checked it, it was not...