Why do ordinarily skeptical people suspend their disbelief when they receive such nonsense?  Perhaps it is because the Internet is a new medium, and since it is growing so fast, a large fraction of users at any time are relatively new to it and haven't developed the critical thinking skills required to distinguish real virus warnings (of which there are plenty -- witness the recent and very real I LOVE YOU virus) from the phony ones (of which there are also plenty -- see below).  The Internet, with its virtually cost-free messaging, is a fertile medium for disseminating all sorts of information.  It is also a mirror of the larger society of which it is part.  Moreover, the casual anonymity it provides virtually invites irresponsible behavior among those so inclined.  So perhaps it's not surprising after all that so much of what circulates on it is misinformation, rumor, or just plain junk.
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*Even the U.S. House of Representatives is not immune from the "gullibility virus."  According to a May 17, 2000, article in The Washington Post, the House this week responded to the silly rumors about this tax by passing a bill blocking the Federal Communications Commission from any attempt to set fees for Internet use.