Can’t join PC to a domain – single label DNS

To enable an Active Directory domain member to use DNS to locate domain controllers in domains that have single-label DNS names that are in other forests, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters
   
3. In the right pane, locate the AllowSingleLabelDnsDomain entry. If theAllowSingleLabelDnsDomain entry does not exist, follow these steps:
   1. On the Edit menu, point to New, and then click DWORD Value.
   2. Type AllowSingleLabelDnsDomain as the entry name, and then press ENTER.
4. Double-click the AllowSingleLabelDnsDomain entry.
5. In the Value data box, type 1, and then click OK
6. Quit Registry Editor.You should be able to join the machine to the domain.