anti-virus software isn't enough
One of the more common questions that I hear from customers is "which anti-virus software should I use?". Unfortunately, there is no easy answer to that. Primarily because your choice of software doesn't matter near as much as your personal habits when it comes to protecting yourself from virus and malware attacks.
Most virus infections today are a result of end-users being tricked into bypassing security measures vs. those measures actually failing to do their job. The key to avoiding infection is to not click on the wrong thing. The key to that is knowing what software you are using, and what a legitimate message looks like.
Take away your own privileges
The safest, most recommended, most secure, and least used method of protecting yourself from malware is not to be logged on as an "administrator". "Limited Users" can run programs, check e-mail, and surf the web. They just can't do things that change how Windows operates. Things like installing printers, setting up networks, and installing software require administrator privileges, but they only need to be done once.
By only using the administrator account when you need to do administrative tasks, you limit what you can change within Windows, thereby limiting what a virus using your account can do. Most viruses are completely benign if they cannot act as administrator. Think of it as only loading bullets into your firearm when you are at the range ready to shoot; it makes mistakes much less painful if they do happen.
The exact steps vary slightly between them, but creating a limited user account in Windows XP, Windows Vista, and Windows 7 is a fairly straightforward process that offers the best protection from viruses. Following the linked walkthroughs, even novice users can create a new, limited user account in just a few minutes.
Know your software
What version of Windows are you running? It only takes a few clicks of the mouse to find out, and it's something you should always be aware of. As of July 2010, anyone using Windows XP (Home or Professional) should have service pack 3 installed, anyone using Windows Vista (Business, Home Basic, Home Premium, or Ultimate) should have service pack 2 installed. If you are not using the most currently available service pack, you are missing out on available security measures.
More important than what Windows version you use, you should know as much as possible about your virus protection software. The 'latest rage' in malicious software today involves counterfeit messages made to look like real warnings from anti-virus software. By clicking "Okay"/"Disinfect"/"Clean My Computer" on these messages, users are actually bypassing their real virus protection and launching the malicious programs. Whatever software you choose for virus protection, you should be fully aware of what the messages and warnings from that software look like, so you know to NEVER CLICK ANYWHERE ON A POP-UP WINDOW IF YOU AREN'T CERTAIN OF IT'S ORIGIN ! If any window ever pops up that you don't recognize as completely legitimate, close it using Alt+F4 (this is the windows shortcut for "close active window").
be suspicious and "surf safe"
Keep in mind that most virus infections are the result of people being tricked into clicking something that allows a program to run on their computer. Most legitimate websites should not require any programs be run to display their content. Be especially wary of "video codecs", "image viewers" or "video players", and only install updates for software directly from the software manufacturer.
If a video you want to watch (or image, or game) says you should install the latest version of Flash Player, or Adobe Acrobat, you should go directly to Adobe's website and ensure your software is fully updated. Similarly, updates and codecs for Windows Media Player should only come directly from Microsoft, not an embedded link that pops up when you try to play the video, or a redirect on a website.
"installed" doesn't mean "protected"
Just because you installed anti-virus software doesn't mean you are currently protected from viruses. If you are using for-pay or trial-version software, your subscription or trial period can run out. Most likely, there will still be an icon in your taskbar even though the scanning software isn't running. If you have anti-virus software installed, you will still see the icon even if one or more parts of the scanning software fail to load and run properly.
Right-click on your anti-virus software tray icon, and choose "run", "control panel", or whatever default action your software uses. You should be able to determine on the window that opens if your software is running, has up-to-date virus definitions, and when it last scanned your computer for viruses.
Find an anti-virus program that works for you, make sure it is up-to-date, and leave it to do it's work behind the scenes.