First, give us the error message. Can't do anything without that.
A couple ideas right off the bat: make sure you've defragged your Steam cache and verified the data.
Otherwise, I can't help you. Detailed information would help some.
Also, check out the Steam support site. http://support.steampowered.com
When I installed it two nights ago, an internet window popped up taking me to the Valve Developer Community website. It mentioned something that had to be added to the target line in the Steam shortcut in order to run the Beta version I just downloaded. I'm pretty sure this is the page: