iTunes doesn't start
You could try the detailed procedure outlined by Apple on iTunes for Windows Vista or Windows 7: Troubleshooting unexpected quits, freezes, or launch issues.

It begins with starting iTunes in Safe Mode and then proceeds to other steps like removing iTunes Preference files, creating a new library, recreating a library and so on.

If you seem to get stuck in this process, I would suggest creating a new library and adding your existing content to it. You would lose your playlists, ratings, etc. Here are the steps:


Hold the Shift key when launching iTunes (from the Start Menu or Task Bar or Windows Explorer).
Click on the Create Library... button and start iTunes.
Drag and drop all your media folders into the iTunes window. If you configured iTunes to organize and copy media, your media files would be under the %userprofile%\Music\iTunes\iTunes Media folder. You can also do this by going to the File > Add Folder to Library... menu option and choosing the folders.