The current version of jQuery that's packaged with core WordPress is 1.8+, so 1.5.1 is pretty ancient.
If you search the contents of your theme files, you should find a reference to "jquery-1.5.1.min.js". You could try removing that piece of code so that it's not loading it (core WP is still loading a newer version, so you'll still have jQuery). It's possible that something in your theme is relying on this specific version, but maybe not.
Definitely loading 2 versions of jQuery is not a good idea though, and since the older version is loading later, it's overriding the newer version.