Repair iTunes Security Permissions
Right-click on your main iTunes folder and click Properties, then go to the Security tab and click Advanced. Use the Change Permissions... button grant to your account (or the Users group) and SYSTEM full control of this folder, subfolders and files, then tick the option to Replace permissions entries... which will repair permissions throughout the library. When complete switch to the General tab, click in the Read-only check box to clear it, then click Apply.
If you don't have the option to change the permissions then use the Owner tab and Edit... button to take ownership from an account with administrator privileges. Tick the option to Replace owner on subcontainers and objects.
Repeat with the media folder if it isn't stored inside the main iTunes folder.
If you've brought over the library from another computer (that it won't go back to) you may also want to remove any "mystery" identities (S<string of dashes & digts>) that have come over from the old computer...
The images above are from Windows 7. Hopefully the system isn't too different in Windows 8.
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