There are more than 70,000 *known* viruses "in the wild" for Windows. By "in the wild" I mean the viruses have been written and released and are spreading from one computer to the next. These viruses can be as harmless as merely slowing your computer down a little bit to corrupting your software to the point that all of the data stored on your computer is lost.
Viruses can attack from many different angles. You can get viruses from visiting an infected webpage, opening a virus-carrying email or attachment, loading an infected program from a disc, or even just by sharing a network with a virus-packing computer... There are some simple precautions that one could take to help prevent the spread of viruses, like don't open emails from people you don't know, and especially not attachments from unknown emails, turning off ActiveX controls on Internet Explorer (or switching to an alternative browser, like Firefox). Not installing programs that you don't remember downloading or asking for, and other things like that. But even the most cautious user - if he is conneceted to another computer at all, especially by the internet, is at risk.
That is why Anti-Virus software is so important, no one should ever connect to the internet without some sort of protection. I'd feel like I have let the users down if I didn't mention that the most secure way to avoid viruses is to use any operating system other than Windows. Mac OS X has *zero* known viruses in the wild, and the number is well under 200 for Linux (I actually don't know of any for Linux, but there may be a few.) Changing your operating system is a tall order, though, and involves a good bit of work, especially learning the new system, and you lose all of your applications, (although most of the open source programs I've reviewed are cross platform, so you can use them on Linux or Mac).
That being said, let me introduce you to a free, open source, and simple way to protect your Windows machines - ClamWin. ClamWin is a front-end of ClamAnti-Virus, or ClamAV. When I say "front end" I mean the part that the user sees, the GUI (Graphics User Interface), the menus, the buttons, the graphics - all of that. ClamWin/ClamAV is a regularly updated anti-virus program that gives users, free-of-charge, a wall of defense against the thousands of viruses out there.
ClamWin can be setup to automatically update the virus definitions on a daily or weekly basis, so you'll never be left with outdated definitions. It can also be automated to run full-system scans of your hard-drvies every night or whenever you'd like to schedule it.
ClamWin intergrates into the Context Menu of Windows Explorer, so the user can scan indivuidual files without having to open the full ClamWin program. ClamWin also has a add-in to Microsoft Outlook, which will automatically delete infected attachment files.