Windows NT knows that you consider the extensions at HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Extension to be documents.
You can define additional Documents, for those extensions that are not associated with an application, by using Regedt32 to navigate to:
Double click the Documents value name in the right hand pane, a type REG_SZ entry, and list the 3 character extensions, separated by a space.
Windows NT considers com, exe, bat, pif, and cmd extensions to be executable.
You can define additional executables by double clicking on the Programs value name at this same path and adding 3 character extensions, separated by a space. This string (REG_SZ) entry, defaults to com exe bat pif cmd.
Example: If you add scr to the Programs string and logoff/logon, you can create a shortcut to a screen saver:
The /S switch forces the screen saver to start immediately. Remove the /S to display a setup screen. You can not use a screensaver that uses a password.
NOTE: A better way to activate a screen saver can be found at tip 0302.