Accessing the desktop of active user loged in in XP

Hi

I am trying to develop an application which has to create a file on the desktop without using the common dialog, i am using the shell command.
now as you know in XP there are different user accounts so i have to create the file on the desktop of the user that is loged in,

Question: how to find which user is logedin at the present time.
for example if we have 5 users in XP which user is loged in so i can create the file on that user desktop

thanks