I don't know of a way to do this - however, in my experience it is not necessary in Win XP. When the laptop starts, just accept that it cannot reconnect, then when the WIFI connection is established, double clicking the disconnected drive should connect it.
However, because I know that the word "should" is one of the most overused in IT, a workaround is to create a shortcut to the shared drive on the laptop (e.g. on the desktop) and click on that after the WIFI connection is established instead of mapping a drive.