Unable to install iTunes

I think I uninstalled apple update software from my computer and now it doesn't appear on the list of installed programs. Is there a way to download just the Software Update? When I try to re-install iTunes It brings up an error message and all of the posts I have been reading say just repair the software update from the control panel's list of Installed programs. If this does not appear on this list how can I repair it or re-download it?

Thanks!