Follow these steps here. http://www.apple.com/support/iphone/assistant/itunes/
If you need to uninstall iTunes make sure it's done in the order the resource indicates.
If the 7 steps above (do yourself a favor and don't skip any steps) don't resolve the issue use this resource http://support.apple.com/kb/TS1538
It can be a pain but there is a solution to this problem.