Ok, to clarify, I now have a valid key and the watermark won't go away.
What you can try is to first, re-run the installer and provide the installer with your product key. When taking this approach, you will not be actually re-installing the product. What would happen is the product key will get registered. Up until the point the product key is registered the product is not aware that you are now a registered user.
If after registering the product key, you still see the 'trial' watermark, you will need to re-boot your system.
Let me know if you need any further assistance.