Hey, I've been coming here awhile and I've found the information very useful and decided to give some back to get out of leecher status. After searching all over for a couple hours I finally found out how to make Call of Duty 4 play in windowed mode.
Press `(~) to get your console to come up.
Next, type /r_fullscreen 0
and then type
/vid_restart
Your screen should be in windowed mode allowing you to chat/browse the internet while you're waiting to respawn.
(Also make sure you adjust your resolution in the in game graphics option to make the window bigger/smaller)