For the server side you can use the built-in Remote Desktop. It's included in all professional (and above) versions of Windows and its behavior is to block the server when a connection is accepted, therefore concealing all activity there.
When a connection is made, the screen is locked and the only way to restore it is to enter the user/password of the logged user. Since client versions of Windows only accept 1 remote user someone "spying" will kick you off the computer, at least revealing that you're being viewed. Conversely, a remote login also kicks the local user. Using a strong password unknown to others with physical access prevents anyone from seeing your activity.
As for the client side, a remote desktop client is also included in all editions of Windows as back as Windows 2000 (and probably earlier too). This client supports most features of the protocol, like drive/clipboard sharing, security certificate validation, remote audio and performance tunning.
Note that this requires to make a direct connection from client to server. For LAN situations this is no problem, but for use over internet you may need to do a port forwarding (open TCP port 3389 by default). If that's not possible or you don't want to do that, there is still the option of using some sort of VPN, for example Hamachi.