Clam Sentinel is a lightweight tool which extends the antivirus tool ClamWin with basic real-time protection.
The program works by monitoring the drives you specify (local, removable or network) for new or modified files, then launching ClamWin to scan anything it finds.
This could generate a lot of extra activity on your PC, but there are various configuration options to keep it under control. You can choose the drives, paths and file extensions to scan, as well as setting the maximum number of simultaneous scans, very helpful when tuning your system for the best results.
It's important to keep in mind that Clam Sentinel is no substitute for the built-in real-time protection systems of other antivirus tools. It has fewer mechanisms for detecting new files, for example, is more easily bypassed, and can't defend itself from malware like most of the competition.
Still, the program's developer does point out that Clam Sentinel great for older computers - in fact, it runs on anything from Windows 98 upwards - and on balance it's a useful extension to ClamWin.
(Please note: you must install ClamWin before Clam Sentinel for the program to work.)