I feel that the primary sources that keep me getting a constant flood of viruses are:

Ebay is huge. As well as all online shopping (auctions to websites.) Anytime you are put in some ones address book - you are vulnerable.

Having a website probably doesn't help... I get very little "blind" contact from the site but I think it generates some of my junk mail

and finally - people who keep 8 billion email addresses in their address-books - or practice "group" forwarding (people that provide services or newletters are the ONLY excusable batch mailings ) are probably one of the biggest problems.

People that forward jokes, freebies, and warnings that 99.9% of the time are just urban legends to everyone in their address-books are a HUGE HUGE HUGE HUGE problem.
This sounds "mean" but if you still buy into all the urban legends and can't resist forwarding jokes, recipes and "a shot at Bill Gates' fortune".... ALL OF WHICH HAVE BEEN CIRCULATING FOR AT LEAST 10 YEARS ALREADY... then I HAVE to assume you probably DON'T understand the consequences that arise from group forwards.
(Of course, if you're one of the people who doesn't bother with anti-virus software, why would you care?)
One click from any recipient on that list, can add all those names to THEIR addressbooks... and since I doubt they knew they just added 30 addresses (of people they never met) to their addressbook... why should I feel comfortable they're going to know or care THAT I CARE whether I am sitting in an address-book of someone who doesn't use anti-virus software????
BECAUSE THESE ARE THE PEOPLE SPREADING VIRUSES.

Sad part is the people who USE Antivirus software get hammered by high volume internet users that DON'T use antivirus software... while the offenders are "out there" in blissful ignorance - practicing "unsafe internet" - and expecting everyone else to pay for their irresponsibility.

I can't feel more strongly about this:
YOU SHOULD NOT BE ON THE INTERNET IF YOU DON'T HAVE ANTI-VIRUS SOFTWARE... PERIOD.