NOD32 quarantines files necessary to run Left 4 Dead.

Add the game folder (default location: C:\Program Files\Steam\steamapps\common\left 4 dead demo) to NOD32 exclusions. Please see the following link for instructions:

"How do I exclude certain files or folders from real-time scanning?"
[ссылка]

Then, verify your GCF Cache files:

1) Start Steam.
2) Navigate to your My Games tab, right-click on the game and select Properties > Local files tab > "Verify integrity of game cache..."
3) Wait until the process is finished.
4) Test the issue