How did make an antivirus software?

Software is programed in any computer language. There is software you use to make other but it depends on the language you are programing!!! Software for used to write software: C/C++ - Dev-Cpp,...

You would have to do a lot of research and learn from software programming. See related link for easy access to programming tools.

You will have to learn how to code. Ether in Java, C, C++, PHP, Visual Basic, Perl, Python, C#, JavaScript, Ruby or Delphi. Then you will understand you can create a program using Notepad. Or use...

Sure you can! Some antivirus providers even provide firewall with them. If you want a stand-alone firewall, then I recommend going for COMODO Personal Firewall.

It`s an application software, because you can download it.There isn`t operating system with installed AV.