WCE400 vs WCE500

Files in wce400 are designed to run on devices that are based on Windows CE 4.X such as Pocket PC 2003 devices. Files in wce500 are designed to run on devices that are based on Windows CE 5.X such as Windows Mobile 5.0 and Windows Mobile 6 devices.

http://msdn.microsoft.com/en-us/library/13kw2t64(VS.90).aspx