Yes, you could have been more clear. The first sentence was about iTunes, which leads to the conclusion that the "windows sidebar" was in iTunes and that "my main computer" was perhaps referring to only internally stored music, etc, and no that stored on external devices.
Ah, well, back to cases: It's probably NOT the iTunes update, as I have the latest version of iTunes installed on July 11th, and my Finder is working just fine. So what, exactly, is missing from the sidebar? Do you have other hard disks, external disks, connected servers, for it to show? And are you saying that the Application, Desktop, documents and Downloads folders, plus your Home folder, even if checked, are not showing up? What version of OSX, what Mac are you running?