Vista Tutorial - Login error

When I try to log in with the Ethernet cable connected I receive an error
message which reads; 'Your logon has been disabled. Please contact your
system administrator.' But if I disconnect the cable it logs in just fine. I
connect to the internet through a network. The only way to logon to the
computer is to disconnect the network cable. I'm afraid to connect wirelessly
because I don't want to have to turn off my wireless card just to turn on the
computer. Do you know why this is happening? Please help as this is an
extreme annoyance.
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Chris R Gates