You have to add 'authenticated users' to the list of users in the security tab of the properties of the file. Then set the permissions to full control 'allow' for that user.
In steps (assuming you have access to the administrators account):
- right click the file
- choose properties
- click the Security tab
- Click edit
- click 'add'
- type in authenticated users
- click authenticated users so it highlighted
- check 'allow' next to full control
That should remove the lock.