Why does net rpc shutdown fail with the right credentials?

Disable simple file sharing on the remote host by going to

My Computer > Tools > Folder Options > View  

And UNCHECK simple file sharing

Also punch the appropriate holes in your firewall by going

Control Panel > Security Center > Windows Firewall > Exceptions 

and CHECK "File and Printer Sharing"

Also on the Advanced tab > ICMP Settings CHECK "Allow incoming echo request"

More info here.

if your user name is of the form domain\uname, try changing the form to domain/uname (i.e., change the \ to / )