I guess you could implement a Service that runs on your server
Walkthrough: Creating a Windows Service Application[
^]
(You can also create a simple console application that does the same job)
Then you can use the FileSystemWatcher
FileSystemWatcher Class[
^]
However, there are some pitfalls.
FileSystemWatcher - Pure Chaos (Part 1 of 2)[
^]
A simple way is to use a timer that fires every 20 seconds and calls a method that checks if there are any files available.
Timer Class[
^]
Either way you have to decide if you should delete the file after processing, move it to another folder, etc.
I hope these links will help you along.