"I am using serial Communication to communicate to the external device know I want to communicate to the PC from the same uC. "

The above finally works out this way : External Device == uC . And now you want to communicate to PC fron this uC. Correct.

You need to come back with more details - do you want to set up the comm at PC end or uC end ? If its the later, then the links given by the members is all that you need. If its the former, well you can start with the simple Hyperterminal or something equivalent to it.

Have a nice day.

raghu