I was curious as to how much spam I actually get in a day, so rather than delete it as it came in, I just let it accumulate for a day. There were 90, most of which came in dribs and drabs steadily throughout the "business day", none at all overnight. This tells me that they are being sourced through a zombie network of infected computers.
Most of these have headers useless for tracing, at least with the tools I have at my disposal since I am not an actual computer security or forensics expert, but they do follow a general pattern that confirms that they come from a zombie network.
The days of "Oh, I don't need antivirus software" are long gone. While the virus/trojan/whatever that is installing the spambots on your computers are likely otherwise harmless, they are sucking up your computer's resources, your network bandwidth, your neighbor's bandwidth, and generally making the online life of everyone less enjoyable because we are receiving the crap that you do not know your computer is spewing out.
There are many reputable antivirus software packages out there, and many that are actaully malware themselves. Personally, I wouldn't touch Symantec or McAfee's home products with a ten foot USB cable, but their business software is good. And there are other alternatives, some you can buy off the shelf at Best Buy, some online, some both, and some that is freeware. Personaly, I use the freeware version of Avast!, and I also periodically spot-check with the freeware version of MalwareBytes (do not have two antivrus packages in always-on realtime mode, or trouble will result, but you can have one always-on and one for on-demand scans).
Now that you've installed your antivirus software, something that is even more important: DO NOT OPEN THOSE MYSTERY EMAIL ATTACHMENTS! You did not just win the national lottery, the defense minister of that African nation is not asking you to help smuggle $50 million US dollars out of the country, and FedEx is not trying to deliver a package to you via email.
Other things you can do to prevent problems is to not use Internet Explorer. While many applications use Internet Explorer as a back-end display engine (because many software engineers are lazy and Internet Explorer is preinstalled on every Windows machine on the planet), that does not mean that you have to use it for web browsing. There are three common and better alternatives, and several less common ones including Apple Safari (I'm talking about Windows users, not Mac users). The three are, in my order of preference: Mozilla Firefox, Google Chrome and Opera. Why? Because unlike Internet Explorer, these other browsers either have built-in or the ability to install add-ons that make your web use safer and more enjoyable by blocking the annoying ads. And on "some" sites (like porn and pirate software sites), you don't even have to directly interact with the ads for their malware payloads to be installed on your computer, merely visiting these sites can do that.