I was looking into the message you are getting when trying to install iTunes and have found a couple of articles that should help you get back up and running. If you are installing iTunes becuase you've just uninstalled the application, then there may be other files that need to be removed. This article named Removing and reinstalling iTunes, QuickTime, and other software components outlines how to completely uninstall iTunes.
Here is the link for Windows XP: http://support.apple.com/kb/HT1925
And for Vista/7: http://support.apple.com/kb/HT1923
If you are just installing iTunes for the first time, then I recommend going through this article named Trouble installing iTunes or QuickTime for Windows found here: http://support.apple.com/kb/HT1926.