There's VNC, which is kind of the predecessor to remote desktop. Once you walk the person through installing it, say over the phone, you just need an IP address and password.
Once you're done helping them, you should probably assist them in making sure that VNC doesn't start it's little server every time with the computer. Any remote control program is a major security risk if left unattended like that. That goes for Remote Desktop as well. They should only be enabled when they are actually going to be needed, and then disabled immediately after.