I find it sometimes necessary to enable remote desktop (RDP) after a machine has left the building. As long as the machine is VPN’d in or somehow connected to the network, I can fire up a couple commands, few clicks, a reboot, and then I’m free and clear.

Open RegEdit (start >> run >> regedit)
Navigate to: HKLM\System\CurrentControlSet\Control\Terminal Server
Change the DWORD fDenyTSConnection to 0 (not 1)
Reboot the machine (shutdown -m \\computername -r -f -t 20)

EDIT:
I forgot to add an important step, namely the step after opening regedit. You must File >> Connect Network Registry, and then type in the name of the computer. THEN you can navigate to the HKLM.