Here's the skinny on what you want to do (though I don't recommend it):
Enable the "Guest" account in the domain (it's disabled by default) if the sharing server is a domain controller
Enable the server computer's local "Guest" account if the sharing server is a member server (i.e. not a DC)
Add "Guests / Full Control" permission to the folder being shared ("just for testing")
The "Guest" account's enbled/disabled status acts as a flag to the operating system that means "Give unauthenticated users access to shared folders / files to which 'Guests' have rights."
To use the "Everyone" group you'll have to set a security option (in addition to enabling the "Guest" account). This is because the "meaning" of the "Everyone" group has changed.
"Everyone" used to mean "everyone in the entire world" in Windows NT 4.0. In subsequent versions of the NT operating systems, a "Security Option" setting "Network access: Let Everyone permissions apply to anonymous users" prevents the "Everyone" group from including anonymous users. Thus, when the "Guest" account is enabled on a stock Windows Server 2003 machine the only shared folders accessible to a "Guest" user still have to contain a "Guests" permission, because "Everyone" doesn't include "Guests" by default.
Once you enable the "Network access: Let Everyone permissions apply to anonymous users" and enable the "Guest" account folders accessible to "Everyone" are really accessible to everyone who can communicate with the server computer.
I strongly advise against actually doing this.