This certainly can be done.

Right Click on "My Computer" and select "Properties"

Click on the "Remote" tab and click the check box marked "Allow users to connect remotely to this computer".

Then, select your remote users. Any user that you would like to have remote access needs to have a password set.

If you are behind a router, you need to foward TCP port 3389 to the PC that you want to have remote access to.

Finally, using your terminal services client, connect to your external IP address, ie xxx.xxx.xxx.xxx

On a more technical note:

You can enable remote desktop on multiple PCs behind your router, and foward different ports to each, ie 3389 to PC1, 3390 to PC2 etc....

Then, you can connect to each PC from the outside using xxx.xxx.xxx.xxx:3390.