"Martin Plechsmid" wrote in
:

The Guests group should be much more restricted then the Users
group, according to documentation.

What documentation says this?
You aren't confusing the "Guests" group with the "Guest" user, are you?

In particular, on my hard disks I never have privileges specified
explicitely for the Guests group. But the user obtains the rights
that are specified for the Users group. As if the Guests group was
a member of the Users group (but it is not).

So, what is the relation between the Guests and Users groups?

In the computer management console (Start - Run - "compmgmt.msc")
Under System Tools - Local Users and Groups - Groups

The description of the "Guests" group reads:
"Guests have the same access as members of the Users group by default,
except for the Guest account which is further restricted."

HTH,
John