Gabe Newell said:

I got involved in the gaming industry when I was working on Windows back in the old days of DOS extenders and config.sys hacked boot disks. I was the producer on the first three releases of Windows. It was common wisdom that it wasn't possible to write a good game in Windows because of, well, unnamed technical reasons. This was annoying, so I decided that we would find the most technically advanced PC game and port it to Windows to show that there wasn't any reason for games not to be Windows apps.



So around the time that Doom shareware came out, I installed it on a laptop and dragged it around everybody's office and said, 'Look, look what PC games can do! This is a lot better that your NES system or your Sega system' , and decided to have some engineers work on porting Doom to Windows.



I called John Carmack and said, "Hey, we'll do this for free". And eventually it became the Doom port to Windows.