I want to shut down my Windows XP box using Samba (v 3.0.2a), I know this is possible.

I made a user called "shutdown" on my XP box and put it in the group Administrators. I enabled Remote Assistance and Remote Desktop in System Properties (I added the user "shutdown" as a Remote Desktop User, too).
Moreover, a user is (auto) logged in at the XP box.

I always get an error-message:

[root@xx:~]# net rpc shutdown -I -U shutdown
[2004/04/17 19:27:42, 0] utils/net_rpc.c:rpc_shutdown_internals(1757)
Shutdown of remote machine failed!

I also tried rpcclient, but I get the error code 5 which means access denied, though I'm sure I'm using the right username and password (I triple-checked this).

[root@xx:~]# rpcclient -U shutdown -c shutdowninit
added interface ip= bcast= nmask=
result was NT code 0x00000005

What can be wrong?