<p>I need to create an application that when a user copies a file into an specific folder, my application will be activated and the file will be changed, also, when the user reads back any file in the folder, changes will also be made to the file.</p>

<p>Is it possible?</p>

<p>I'll use .net for this.</p>

<p>I think the folder C:\WINDOWS\assembly is somehow like this?</p>

<p>Any help will be appreciated!
Thanks!</p>

Well, I'm sure you could write an explorer extension that would do that, but the more typical method that I know of is to write a windows service that monitors the directory in question using the FileSystemWatcher class to monitor events.
The service can then perform the requested actions (or spawn an executable to do it).
One caveat I have noticed is that sometimes the file watcher can miss events if a lot of activity occurs at once (as of .net 2.0 - haven't checked 3.5).
If you use that method, make sure you check the directory contents yourself when you receive a change notification rather than relying on what the filewatcher tells you. I ended up combining the filewatcher event handling with an infrequent poll just to make sure I didn't miss anything.