OK, RDT is Remote DeskTop. As Far as deciding plattform, I use xp pro, use it for my self only. I think there maybe be some limitations to how many simultaneous connections the pc will except based on licensing. If this is a true office/work envirnment, and may have multiple connections at a time, I would use 2000/2003 server. If its just one or maybe two users using vpn, u could get away with xp pro. Below, is how to set it up.
On Dedicated Server:
Goto properties of MY Network Places-->
Click create new connection-->
Click Next-->
Click setup advanced connnection-->
Click accept incoming connections-->
The next screen is where you would select dialup modem if you were using Ras through dialup, But from the sounds of it, you want access through internet. Make sure the modem or direct parallel is unchecked.-->
Check allow vpn connections-->
Check the users that will be accessing the network in the user list. If they dont exist, you must create them.-->
Here is where you will highlight the TCP/IP protocol and select Properties-->
Make sure allow user to access my local area network is checked. Below that field is where you specify a valid IP address range that is not in use on your internal data network as these are the ranges of address the remote users will be assigned when connected to the network-->
Click next and then you are done configuring the server.-->
Make sure the firewall is turned off on the pc no matter what platform you decide to go with.-->
You must forward port 1723 on the outside firewall to the Ip address of the server.
Configuring Client PC:
Goto properties of MY Network Places-->
Click create new connection-->
Click Next-->
Click connect to my working place using dialup/vpn-->
Click Using VPN Connection-->
Enter the label or name of this connection-->
Click Do not dial Initial connection-->
Enter Internet IP address of the Office or Company (ISP IP address)-->
Select which option is necessary for needs, For a single user or anyone who uses pc can access this connection-->
Click next and you are all done.
You will then find the connection listed in my network connections.
This will work great. By default, the vpn uses 128bit encryption which should be plenty. If its not, you can adjust that in security policies I think. I never changed it but im sure you can up it to 256bit. If you need more assistance, drop a message here. I check back every couple of days or so... Good Luck