identify file used by other applications or process.

Question

hi,
can any one help me out with this problem.I have been googling for past few hours but couldn't find a answer.
I am doing a application which requires to identify which file is used by a external application or process.we can identify the modules associated with the process....similarly is there any mean of identifying the file used....
eg: if i have adobe photoshop running than is there any means by which i can know which file is it using(or working on ).

guys do reply its bit urgent.....

thank you.