Why don't you have anti virus software and a firewall?
I am using McAfee. It is automatically updated every day, sometimes several times a day. I have never had a virus or worm infect my system.
So9
I do now. I had McAfee at that time and it was apparently useless. I since installed Norton's which did a great job of cleaning up the mess on my computer. I thought it was toast at that point. I also have Zone Alarm, but I am a techno-moron so I really don't know how to effectively configure and use it without locking myself out of places I want to be.