ExeWatch is a free tool which raises an alert when a new executable appears in any folder, on any local or removable drive.
If you're installing an application then this won't be any great surprise. But if you visit a website and it forces a download, say, or you unzip what you think is a data file but actually contains executables, it could be helpful to find out more.
Whatever your interest in the program, it's very easy to use. The download is tiny, and there's no installation: just launch ExeWatch and it runs in the background, monitoring your PC.
When a new executable is created, ExeWatch plays a beep and displays a visual alert (four boxes, one in each corner of the screen). Double-clicking its system tray icon displays any new detections, while a right-click "Open History" option displays everything the program's ever noticed.
Normally this is for information only, but enable "Panic Mode" and ExeWatch can also rename executables as soon as they're detected. Under typical circumstances this is a very bad idea - you could cause all kinds of problems with legitimate programs - but if you're sure something dangerous is happening then this might help bring it to a halt.
If you find ExeWatch raises too many false alarms then you can now also add exclusions, folder trees where new executables won't raise an alert. All you have to do is add their paths to the exclusions.txt file in the ExeWatch folder (a couple of sample folders are in the file already to show you how it works).
A simple, lightweight system monitor, maybe useful on PCs where you're don't typically install new software.