The location that this defaults to is browser-dependent, and as far as I can tell, there is no way of setting a preferred working directory.
Also, it would be very difficult to guess which directory you would want to start it, considering that you may get a user with any of a number platforms.
From http://www.irt.org/script/1154.htm:
Because of security implications (scripts setting the value, and then attempting to retrieve files of your computer) the value of the fileupload form field is read only. Therefore you cannot set, or reset the value.