lots of people run their computers completely unprotected. anything that wants to infect their system can. usually those who don't bother protecting their systems are the same type that are too lazy to bother verifying if that awesome file link they just clicked on will infect their system.
some people will say "well, i just double clicked the file, and nothing happened!" - well, it may look like nothing happened, but in all reality perhaps that file just extracted several more files on your computer, and just loaded a key logger, a virus, and possibly a back door program so everyone can look at all your files. way to go! it did all that behind the scenes, without showing you that anything happened. after the user double-clicks it a few more times, they just go ahead and delete it. it's too late though, for it has already delivered its payload.
some people install a virus scanner, but then never update it. having a scanner with definition files from 2004 has almost zero effect on everything that has come out then.
its possible to see certain brands of virus scanners advertised and sold at local stores. the two biggest are Norton AntiVirus and McAfee AntiVirus. obviously these are the most popular. not because they are the best, or because they are even good - but simply because they are advertised the most. they spend money to advertise, they make money, and can spend more to advertise even more. clueless people buying these products just perpetuate them being sold to even more clueless people.
they cost a lot and require you to keep paying yearly fees to stay up to date. they usually take up a lot of disk space and system resources. they usually present you with false information as well. false information to make you think you are more secure, or false information about how it has just done something to protect you.
they may end up letting something slip through, or they may mark valid files as "dangerous".
there is also the whole "performance" issue. many people pay top dollar for their computer, and then pay a lot for one of these crummy virus scanners. i've always noticed speed reductions with the popular virus scanners - it seems like someone has done some benchmarks on the issue now, testing many virus scanners and other applications.
you can check out that information and the benchmarks here:
http://www.thepcspy.com/articles/other/what_really_slows_windows_down/5
at the top of the list for slowing down systems the most:
- "Norton Internet Security 2006"
- "McAfee VirusScan Enterprise"
regardless of what virus scanner you get, make sure you pay attention to what files it marks as "bad". many virus scanners will mark legitimate programs and other software as a virus, trojan, or other malware when the file may actually be perfectly safe.