If the comm port does not work then you are a bit stuck. Have you got the right pin-outs for the console cable, and also the right speed set.
To check whether the port is working, connect the console cable to the 450, run hyper terminal @ 9600 8 1 none settings and power off and on the switch. You should see the POST tests running on screen. If not then the console cable is incorect or you have the wrong settings or the port is stuffed.
If you do see the tests performed, at the end of them to login you sometimes need to do a CTRL Y or CTRL R to get into the main menu, you can then alter the IP settings from there. If alll this fails then I suggest you go back to the person you brought it off and ask them for the IP details and password if one one was set
Hope this helps