Close steam.
Delete "left 4 dead 2.ncf, left 4 dead 2 client.ncf, and left 4 dead 2 common.ncf" from your Steamapps folder.
Restart steam.
Right-click Left 4 Dead 2, then click "Install Game."
Also, if you're using avast! Free Antivirus, disable it before playing for optimal results.
This will redownload just the client and common to your system and will read over the already downloaded Left 4 Dead 2 files, so don't worry if you've got a low bandwidth cap, or one at all.