Can the Nimda virus cause a network to work just fine and then slow to a crawl as soon as you plug it to an active Internet cable modem connection?
It is possible that if you have Nimda, plugging in the Internet connection can kill your bandwidth. Remember, Nimda transmits to many others, thus upon reconnecting your mail server and others, it will start to transmit. Although Nimda is suspect, there are other things you should check, such as a misconfigured mail server (SMTP and POP), auto update software (virus, content checker, Microsoft patch) and a bad network interface card.
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WhatIs Definition: Network interface card
This was first published in August 2002