Is there a way to run rmiregisty and my RMI servers in the background under Win32?

You can make use of the Windows JDK utility javaw to fire-up the Java interpreter as a separate process and run rmiregistry and your server in the background.

For example, you can start up rmiregistry in the background as:

javaw rmiregistry

However, do note that this will preclude you from seeing any of the diagnostic or error messages that would be printed to the console by default.