Clam Sentinel is a system tray application that detects file system changes and scans the files modified using ClamWin (require the installation of this antivirus).
Also detects new drives added (like usb pen) and monitors these units until the program is closed or until the device is disconnected.
For Windows 98/98SE/ME/2000/XP/Vista and Windows 7: here is a list of some features.
Clam Sentinel is freeware and open source project hosted on SourceForge: Clam Sentinel on Sourceforge.
Clam Sentinel is developed with Delphi (D6).
Tested on Windows 98SE, Windows 2000, Windows XP, Vista and Windows 7.
Here two short guides:
The Clam Sentinel Program Description And Setup; Getting Better Protection With Clam Sentinel.