Showing Directories(folders) in Form

Hi friends,
I just want to show the directories(folder) in the form. e.g i have a directory 'a' which is in D drive. When i click on 'a' it show me all the dirctories in 'a'  and so on until a didn't reach to a specific file or folder. I'll be waiting for your reply. Thanks in advance.
Zeb-ur-Rehman.