Just "shooting from the hip" here, because I never have actually done it, from windows to unix. On linux to linux, you can ssh( or telnet) into the other machine, export the display(after setting the Xauthority), then run a Tk app remotely. I havn't tried it in a long time.

Going from windows though, you may need an X emulator running on the windows machine, and use "putty" to log in to the unix machine. SSH will usually handle the "export display" automatically, if configured right. I don't know how putty would handle that. Just ideas for you to consider.