Instead of using TCP/IP as your network protocol, install NetBEUI...it's nearly foolproof. Also, is it safe to assume that you've created new users with the correct passwords on each computer? Have you shared your drives? Have you installed the "File and Printer Sharing" service on both computers? Have you set permissions for each respective user under Security? OK, my brain is full now...I'm going home...

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Sometimes computer problems require extreme measures. Our hero, the Intrepid Spaceman Spiff, sets his blaster on "deep fat fry..."