Updating your AV software
To keep your system virus-free requires a little more effort than just installing the antivirus program once then forgetting about it. New viruses are released into the wild every day, and in recent years the propagation of worm-building kits available to malcontents over the Internet has led to an increase in the rate of virus creation. This, combined with an increase in public knowledge of security problems regularly found in major software and operating systems, has led to an even higher rate of new virus discovery. No longer does a virus have to be skillfully crafted, but slightly different viruses can be created by almost anyone and re-released into the wild. This means that in addition to the product out of the box, regular updates are required.
The good news is the vendors know this and include automatic updating tools in their new products. For the PC user, this means updating your AV is usually as simple as selecting "Update definitions" from the software menu. Some also include scheduling applications that allow background automatic updates of the virus definitions and scan engines that offer helpful reminders when definitions are out of date. A common approach is for the antivirus program to check the latest version available via the Internet and to prompt the user to press a button if their definitions need updating (as can be seen here). Updating is quick and painless and usually requires no more than clicking "Yes" a couple of times.
The bad news is that out of the box products usually only include updates for a specified time period and then require a subscription to their signatures service to continue the updates. Generally they provide a year's worth of free updates with the purchase of a commercial product but will require an ongoing annual fee to keep the definitions up to date after that time. The antivirus vendors now have teams of experts working around the globe 24/7 to respond to new viruses and charge a premium for the service. Vendors provide updates for each product version for several years until they officially end-of-life a release. Some offer updating to this year's release along with another 12 months of updates for only a few dollars more than signing up for another year of signatures.
Over the last three years the time taken to release new signatures has dropped so significantly it is reasonable to expect your antivirus package to keep you protected from any virus you come across no matter how new, as long as you always make sure to update your signatures before downloading your e-mail or running any new executables.
Bottom line: If you have IM/email integration then you won't be able to download any infected file to run.