I feel I want to answer, but I'm not sure what the question is. I spent quite a lot of time installing, running and uninstalling antivirus programs - usually for other people.

If you want to know how they work, can I suggest you look at the Wikipedia article?

http://en.wikipedia.org/wiki/Antivirus

It's a personal opinion, but for the last 2-3 years antivirus programs have been almost as bad as the programs they are designed to prevent. They are bloated, intrusive, hard to control and hard to remove. I hold the view that people should be allowed to use a computer only if they show sufficient intelligence and aptitude. That would bring Facebook's value down a cent or two, though.

AntiVirus software gets it's name from the virus in life. A bit of code is introduced into a programme and has the effect that the program will not function normally. The problem is that there are many viruses and the code that is introduced often looks like the desired code in a program.

Even more fun is running two AntiVirus programs together. If the virus database is not constructed well, the other program will recognise the database as an intrusion.

You can actually get signature files that you install to make sure that the program you have installed is working properly. The files themselves are harmless.

Computerviruses are illegal, a nuisance (partially because you need AV-software, but there are bright sides. Sometimes they are fun, they seem mostly to be oriented against Microsoft products and I can not help but be amused when a complete government agency is shut down, temporarily, because their computers have been infected.