I have a application written in PyGtk. I need to convert a particular line number into its corresponding window co-ordinates in a GtkTextView. How can this be done?
It can't be done, in general.  If you're talking about a line of your pygtk program, then you would need an API for your IDE (development environment) to find out where a particular source code line is currently displayed on the screen, if anywhere.  If by "line number" you mean something else, then as Ignacio says, you'll need to provide a lot more detail.
Convert Line Number Into its corresponding co-ordinates