First, you need to allow the compter you want to remotely control to be controlled. Launch Remote Control Preferences, and allow Ubuntu to be remotely controlled. You can also set a password if you want. You can now control that computer remotely from another Ubuntu computer. Make sure to choose the VNC protocol when connecting to the computer. If you would like to control Windows 7 from Ubuntu, you need to choose the RDP protocol (You will need to have remote control allowed on the Windows 7 computer).