Question:
Is there a possibility to get notified by the operating system when a file in a certain directory is opened, closed? I need those notifications in realtime.
Answer:
Yes, Microsoft provides an API for this:
Win 32 API
If you use an older version of Delphi then you could use the free RXtools (aka 'RXlib'). They include a directory monitoring component which will get notified with an event. The component's name is TRxFolderMonitor and it works very well. The source code is available but I think it has not been ported beyond Delphi 5.
See the link above how to install RXLib.