Deppends on what program. Bios programming is very low lavel, you program it direct in the chip, I do not not excatly what programming language is used, But for low lavel programming I used once Assemble Programming (Very tough) Operating systems are programmed in different languages, For example, C++, C#, Delphi and many more. I think windows is compiled with C programming. Others like web page and so on are in html, css, php, java, asp and so on and so forth.