If you wish to play L4D at the LAN you need to have steam installed and set to offline mode before the LAN
You don't need any games just steam with a account and it able to run in offline mode
Step 1. If you do not already have steam go to the steam website
http://store.steampowered.com/ and click the link on the left side (big green thing you can't miss it) and install it
Step 2. Make a account if you do not already have one (also do this for all computers you may also be renting out or may want to play L4D on at the LAN)
Step 3. once you have it installed and a account login and go to settings > Account and down the bottom there will be a tick box(don't save account credentials on this computer) MAKE SURE THIS BOX IS
NOT TICKED
if you tick it you will not be able to use offline mode at the LAN and there will be no L4D for you
And if you already have your steam set up do not worry about it
Any questions mistakes please post and tell me about it