I have 3 "client" computers, on which the mentioned user is administrator:

CPU1: Win Vista 32-bit -- User: Domain\User1    -- IP: 192.168.42.1
CPU2: Win 7 64-bit     -- User: localhost\User2 -- IP: 192.168.42.2
CPU3: Win 7 64-bit     -- User: Domain\User3    -- IP: 192.168.42.3

And a "target" computer (the one that I want to shutdown from the three others):

TGT: Win 7 64-bit      -- User: localhost\User4 -- IP: 192.168.42.21

I'm trying to shutdown TGT with the following command:

shutdown /s /m \\192.168.42.21

It's working from CPU1 (meaning TGT shuts down), but from CPU2 and CPU3 I get the following message:

Access denied. (5)

What am I to understand? What should I do to get it working form all of my computers.