The best way to avoid exposure to emailed viruses is to keep those messages out of your inbox entirely by getting yourself a spam filter like MailWasher, IHateSpam, SpamPal, or POPFile if your email provider has been having a hard time keeping down the daily deluge. If your computer is exposed to viruses and other malware, you could become an unwitting helper in the spam flood. How? Spammers and virusmakers have joined forces in their quest to steal money by creating trojan horse programs that give hackers a back door into your computer. Once inside, the hacker turns your computer into what's known as a "zombie", and it takes its place as a node in a potentially-enormous botnet: a network of hijacked computers that spew out endless streams of spam.
The nasty part is that your computer could be hijacked and you wouldn't even know it. Except for the fact that programs seem to be running much slower than usual, and your Websurfing has slowed to a feeble dogpaddle. You might get a terse message from your admin or ISP telling you that they've cut your computer off from the network until you've dealt with the problem. You might suddenly find that emails you try to send to your friends are bouncing back with arcane-looking error messages because networks have tagged your computer as a spam machine. To find out if you're part of a botnet, you should plug your IP address into DNSstuff's Spam Database Lookup. If lots of places have your IP address banned, chances are good that your computer has been hijacked.
The worst part is that unless you're extremely computer-savvy (in which case this article is all old hat to you), you'll have to wipe your computer clean and reformat your hard drive to dezombify your computer.
The same thing might sadly be necessary if you get infected by certain types of spyware adware spread through websites. Since adware is written by jerks who only want to steal your information or bombard you with advertisements, none of it is written with any concern towards what it might do to your computer. So, adware tends to hog system resources, mess up the function of your web browser, screw up your mouse, etc. Severe infestations can render your computer completely non-functional.
The worst of the worst come from dodgy porn and filesharing sites; avoid those if possible, or visit them on a computer you don't use for anything else. I once bought a computer from an acquaintance with indiscriminate surfing habits, and when I was giving it the once-over I discovered loads of malware, including a keylogger program that was ready to transmit all the usernames and passwords I typed into my online accounts to a computer in Romania. Did I reformat the hard drive before I used it for anything? You betcha.
If you don't want to switch to Linux or a Mac, the simplest thing you can do to avoid 80% of all the spyware and adware out there is to use some other web browser besides Internet Explorer. Malware writers, like most thieves, are fundamentally lazy; they tend to churn out programs to plague the browser that most people use -- Internet Explorer -- and ignore the rest. I like Firefox myself.
If you think you've got spyware on your computer (having weird web browser malfunctions and strange random ad pop-ups is a definite sign of infection), you'll need to look for it remove it. The problem is, there are a fair number of programs out there that purport to find and remove spyware when in fact they simply install more on your computer. The programs I've used that do a decent job are SpyBot and Ad-Aware but even they sometimes can't cope with the trickiest adware trojans -- you may have to resort to a complicated program called HijackThis.
If that fails, you'll have to wipe your hard drive and start over from scratch.
And that's when having up-to-date backups are absolutely crucial.