A possible reason is either a Domain specified Group Policy or a Local
Policy setting.
Use the Resultant Set of Policies mmc snap in and see if something is
enabling User, Configuration, Administrative Templates, Control Panel,
Printers, Prevent addition of printers.
To start the Resultant Set of Policies mmc snap in:
1. click Start, Run and key mmc /a (press Enter)
2. click File, Add /Remove snapin...
3. click the Add button
4. double click Resultant Set of Policies, click Close
5. click OK
6. select the Resultant Set of Policies in the left pane
7. click Action, Generate RSoP Data ...
Follow through the prompts in the Wizard.