The following article has been helpful for many people who are having this problem. You have to make sure you remove not just iTunes but a few other programs like Bonjour, and Quicktime before you can properly reinstall itunes after you try to uninstall it. The registry keys must be cleaned up properly or the attempted reinstallation thinks the old copy is still there. ( even if it doesn;t show up on your programs list. )
iTunes can be very difficult to uninstall properly because of a number of registry keys left behind by other pieces of software that it works with that also need to be removed. The article explains what you have to do step by step if you can't uninstall iTunes using the ADD/REMOVE programs. If you don't uninstall it properly you can get all sorts of errors when trying to install a new copy... even if you try to write over your existing copy. It saved me hours of aggravation and I did not lose my music!