The_Unbeliever
Honorary Master
You can but to have more than 2 concurrent connections you need to have a application sharing environment. Besides, if you're insistent on using open source there will be no such thing as remote desktop.
SuSE Linux (and others) are able to connect to Winders PC's with remote desktop.
You can use VNC on Linux and Windows should you need to remotely access a Linux (or pre-WinXP) PC...
SME Server also offers a complete PPTP VPN server which's 100% Microsoft client compatible. I have set it up and used it previously, works just as well as any Winders server - plus SME server is free.