VNC, it's an option on OSX Server (as well as OSX regular)

for PC software:

RealVNC - RealVNC remote control software
TightVNC: VNC-Based Free Remote Control Solution

"But it's not secure" you say - ok - then:

setup a SSH session and SSH tunnel for VNC then you'll have the extra security you need.

here's a link that will lead you to a screencast on setup on the server (it's a bit of an old one, but you should be able to get enough information to make it work):

Mac Server Series: How to create an SSH tunnel for a secure VNC connection » Free Mac Blog

and on the pc you can setup a few things to create your side of the tunnel like:

putty ssh to tunnel the vnc, here's a link on how to do it: (old but still good - you can ignore the smoothwall aspect if you're sitting on the same network, or modify the instructions to fit your own firewall/router - plus you don't need to ssh to port 222, port 22 is the standard ssh port)

Smoothwall - Tunnelling VNC over SSH with PuTTY

or you can look for some other puttyssh vnc directions that may more fit your network model:

putty ssh vnc tunnel - Google Search

here's another tool you can use to setup a vnc over ssh tunnel:

ssvnc: SSL/SSH VNC viewer