run on a task if a file is available

[plord]

        Perhaps I am being a little bit dense here but 
I am a little bit stuck. I want to perform a task conditionally
if a file is present (actually I want to copy this file) but
otherwise I just want to continue on with the build process. 

        Now the available task allows me to set properties on 
this basis, but I can't see how to go on from here. Is there
any generic way of performing some sort of conditional action? 

        Phil