From Turingtest2: Right-click on your main iTunes folder (normally in User's Music unless you have relocated it) 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. Note also that the Administrators group should normally have full control, though iTunes should work properly either way. When the permissions have been applied switch to the General tab of the folder properties dialog, click in the Read-only check box to clear it, then click Apply.
He also noted that anti-virus software can also create this error.