User Log In Problem

Got a strange problem on an XP Home Machine.

When the machine first starts up, it gives you the option
to login as one of four people.

Once you have logged into ANY of these people, they are
subsequently unable to either:

Log out
Switch Off
Restart

However, if they use the Switch User option, they can
then shut down / restart from the user selection screen.
Also, if another account is logged in, they are able to
shut down / restart from the that account, but not the
first account.

Any help would be greatly appreciated.

Thanks
Andy