If you want proper online play, buy Left 4 Dead on Steam (often $2 during sales). The ZIP install is only useful for offline/LAN nostalgia or testing.
Installation Verification and Status Report: Left 4 Dead (Build v1027) file left 4 dead v1027zip install
Keep boomers off your tail, and happy hunting. If you want proper online play, buy Left
Find where Left 4 Dead is installed (usually Steam/steamapps/common/Left 4 Dead for Steam users, or the custom install path for others). If you want proper online play