Desktop Background Changer (DBC) is a utility to regularly change your desktop background. It can choose images from multiple directories, and its update interval can range from 1 second to 999 days. It can handle multi-monitors independently, and it can even show a grid of images per monitor.
DBC runs as a "tray icon" application in the Windows System Tray. If you want it to run each time Windows starts up, you can drag a shortcut to it into your Startup folder.
DBC requires no installation, and full C# source code is available. The only prerequisite is the .NET 2.0 Framework, which is freely available from Microsoft.
DBC is CharityWare. If you use it, I ask that you make a $5 US donation to the charity of your choice.