I think Little Hawk made an excellent point, and I'd like to put it in bold print:

Main discussion - I am routinely suspicious of any message warning me about a virus. Such messages frequently are viruses themselves. I prefer not to receive emails warning me of viruses. If I do receive one, I call my server right away, and they have been very helpful. Due to their good advice, I did not follow the very sincere instructions sent to me by a well-meaning business associate...and delete a useful and necessary file off my hard drive (supposedly the file was a virus).

It seems to me that I get virus warnings most often from people who have very little technical expertise. I usually follow through and check out the warning so I'm able to provide correct information - but most often, the virus warning e-mails I receive are just a well-intentioned nuisance.
-Joe Offer-