For windows 7:
1) Open itunes and right click on the file you want to change
2) Right click on 'Show in Windows Explorer'
3) Right click on the file and click on 'Properties'.
4) Click on 'Security'
5) The first item is probably 'System'. Click on 'System'.
6) Click 'Edit'
7) All 'Allow' boxes should be checked. Click Allow at the top of the column - it will check all boxes automatically.
Cool Do the same for all items listed, ie: Administrator, and all Users etc.

You are done.