Portal: The First Slice Demo Glitch
Solution 1:
Some of the files probably got messed up when you were downloading or something. Go to your steam library, right click on the demo and click "properties", then go to the "local files" tab, and then click "verify integrity of game cache" fixes a lot of issues like that for me.
This only works if you're playing via steam though.
If you dont mind losing your progress to that point, you can just uninstall and reinstall and it should work fine.