These instructions are specific for GNOME2 (the usual desktop environment on our computers). Note: the screenshots are not identical to what it could be on your screen, but try to follow the instructions.
In Gnome Session (System –> Preferences –> Remote Desktop)
Here you can do all the settings that you want to secure your machine over network.
You have to unselect “You must confirm each access…” for obvious reasons. Unselect “Automatically configure UPnP…”. The firewal will block such requests. The connection is not encrypted and it is safe to use it this way only within our network. Follow this manual to learn how to connect to you desktop from outside our network. Needless to say, put a password.
Make sure you turn off your screen when you leave, since everything you do remotely will be visible on your desktop computer. If you feel comfortable using Linux there are alternative configurations that will avoid this problem.