The best I could find you are some file and driver updates for the Pocket PC 2000 OS that you're running now. They're from Casio Worldwide.
http://www.casio.com/support/driverd...ssoftware.html these will update your device but not the operating system.
What happened is about a year or two after the Pocket PC OS was released, Microsoft decreed that only products with ARM compatible processors would be supported. Casio's answer to that was to release the E-200 which runs on a StrongARM processor. But by the time Pocket PC 2003 came out Casio had left the consumer PPC market and switched to their own OS on top of WindowsCE.
It would take major hardware replacements to upgrade an EM500 to any other OS.