You could buy the
id superpack on steam. Then you'll no longer have to worry about CD keys and crap. And you'll get the Doom Trilogy as well as the entire Quake series, Wolfenstein games, the Hexen games (never tried em), and a couple other things.
EDIT:
That last link was a lot of money($70), and maybe you don't care for DOOM, or Wolfenstein, and other stuff.
They have the
Quake Pack for $30.