This happens because, somehow the iTunes Library.itl file got set as read only. You can fix this by doing the following steps:
1. Open the My Documents folder.
2. Open the My Music folder.
3. Open the iTunes folder
4. Right-click the iTunes Library.itl file and select Properties.
5. Uncheck Read-only.
6. If you have a Security tab, select it. Ensure that your account has Full Control permissions to the file. If you don’t have this tab, ignore this step.
7. Click OK.
Now you should be able to start iTunes without the error.
Article ID: 55, Created On: 9/19/2011, Modified: 9/19/2011