This is something I did a few years ago and has been very useful. I'm a *nix admin that occasionally needs to log on to a Windows server. I used rdesktop for a long time to do this. I wanted, though, a nice GUI that would allow me to pick which server I wanted (and an excuse to play a bit with Tcl). This is the script I came up with to do just this:
All you need to do is plugin "real" values for the server and addresses. You can also auto-login by giving values for the username and password (may also want to change the geometry of the window unless you have a big screen).
I also have bash aliases for these, but that's a different post...