I just finished downloading iTunes, but when I clicked "Run" Windows indicated that it was not a valid WIN application and I lost the download. What can I do?

I've been having a lot of trouble with iTunes lately. When I would sync my iPod to iTunes, many of the most recent songs I had downloaded would be removed from my playlists and their respective play counts wouldn't be updated. I had thought that updating iTunes would fix my problems, but each time I tried to download the update, nothing would happen. The dialog box that gives you the option to open or run the program would not appear. Finally, after referring to the Apple Support Community, I was able to find a link to download iTunes that allowed me to do so. But when the download was complete and I attempted to run the program, a Windows error occurred that indicated iTunes was not a valid WIN application. It gave me no other option but to click "OK" and when I did, I lost the entire download. I hope that there is a way to fix this problem.

Thanks for reading.

Kristen