You can definately turn it on remotely.

The key "HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server" has a setting "fDenyTSConnections" that has a DWORD value. Set it to 0, then you can use RDP.

Then just add users to the local group 'Remote Desktop users" and your good to go. if the user is an admin they do not need to be added.