Controling app from outside

[LaBBa]

Hi
i would like to know what Api are in used if i want to create an app
that would control another app..

like all of those KazaA find more sources.. they all control kazaa and sending
to it the inside command of kazaa to find more sources...

i have an app (a Dialog) that i want to send it like it was clicked on the "OK"
button

how to do so. ?
if you have source codes (VB is more then welcomed!!!) i will be more then
thankful.

LaBBa