I intend to download Steam, but I already have Half Life, Team Fortress Classic, Opposing Force and Richochet. Should I download the Steam installer on its own, or the full Steam installer?
Simply download the 700 kB installer.
When you install Steam, it will automaticly detect your WON installation and convert it to Steam platform files, but I suggest that you re-download all games through Steam.
That is, if you really want to use Steam, otherwise don't bother. Program has more bugs and errors then Windows has/have . . .