Do you have an anti-virus program such as McAfee or Norton's on your PC? If so, good for you! Now - are you keeping that anti-virus program up to date by downloading its virus definitions monthly? NO? Join the club, you are in plentiful company.
It isn't sufficient to simply load an anti-virus program onto your PC - you must refresh its list of known viruses or, as time goes by, it becomes more and more out-of-date in its ability to detect viruses - notably the newer ones.
Say you purchased and installed McAfee Anti-Virus last Spring. Part of the program are three files: names.dat, clean.dat and scan.dat. These are the 3 data files containing the virus definitions for your McAfee anti-virus program. (Other companies anti-virus programs will have their definitions stored in a different manner.) The package you bought last Spring could have been created at the factory last September. Thus your McAfee Anti-Virus, right out of the box, would know nothing about viruses created since last Fall! ACK!
There are 2 main ways to update your virus data files (1) download them from the companies web page or (2) have the company send them to you on floppies. There might be a small yearly charge for a subscription to the mailed floppies. Both means are discussed in your documentation. It is up to you to place these files in the correct directory, though æhow-to' instructions should come with the data files explaining what to do. Don't want to deal with all of this? Call your family or neighborhood tech.
Recently while installing the Norton's September data files from a floppy disc mailed to my customer, I discovered that the version number of his Norton Anti-Virus program is significant. Their older programs needed to be upgraded before the new data files could be installed. The upgrade for my customer's older program was downloaded off the net and applied successfully. phew!
Whether you purchase McAfee or Norton or any other company's anti-virus program is up to you. There is a lively debate in PC magazines of the relative merits of each brand. McAfee and Norton have the majority of use due to their reputations and heavy marketing!
Moral: Choose one anti-virus program and keep it updated!