Go to
http://www.realvnc.com and download the "Server and viewer" package suitable for both computer's OSes. Then install and make sure while installing that you check the boxes that talk about 'Starting VNC as a service' etc.
Once installed, it will pop up with the control panel box so that you can change the password, and other settings if you so desire.
Complete the install on both computers.
Then, all you have to do is from your computer, type in the IP (or hostname) of the computer that you are wanting to connect to, and hit enter, and type in the password that you set and voila! you can see, and control the other computer.
If you know what remote desktop connection or GoToMyPC.com is like. This is similar. It's just like you're sitting at that computer using it's keyboard and mouse. Even though there's minimal lag, it's still not enough to watch video or play games though.
HTH,
Dave