Is there any way to invoke an Window Application (for eg say calc.exe) from a Unix Server. I think we might be needing a Windows X Server running. I am not sure how to invoke an application from the unix server. Any help on this.
If you intend to run calc.exe ON the Unix server, you'll need some sort of emulation/translation software first. Assuming that your Unix server is capable of executing Linux binaries, one program that comes to mind is VMWare Server. It's free, you don't need an X11 environment to run the server program, and you can connect to it from any Linux or Windows client. qemu can also work in a similar fashion over VNC, although its performance is generally below VMWare's.
Alternatively, you can run X11 on the server and use Wine to run it in real time. You could then connect to the server either through an ssh tunnel, or through a remote desktop environment (like VNC).
Wine has recently come a very long way. Version 1.0 brought new levels of usability that I hadn't expected from them (I half-assumed that it might someday may minor improvements but that its stability would never be really usable with larger scaled applications -- glad I was wrong!). The latest version is 1.1.5 which you can grab at http://www.winehq.org.