Hey man, are you sure that the telnet service is enabled in control panel? I know you said you have the settings, but make sure the service is enabled.
Go to control panel, and then programs and features. On the left hand side click "turn windows features on or off", and make sure telnet client is ticked.
The just go start > run > cmd. Type in "telnet modem IP" without the quote marks and follow the steps on the Tripwire site
EDIT: Also, what model Modem is it? Is it a modem by itself, or a modem/router? Have you tried googling "Default *** Modem username and password?"
Edited once, 20/3/11 - 11:48am.
Posted on Sunday, 20th March 2011