It sounds to me as if both computers have got the same IP address, Can you go to Start, Run, type in CMD and click OK. In the resultant DOS window type in IPCONFIG /ALL and press enter. Do this on both PCs and tell me what IP address you get returned.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.