I found this after searching google, might be of help
This kind of thing is usually caused by an interruption in the update download which causes an error.
A possible fix for this problem is to navigate into your Steam folder and delete:
Make sure your steam is closed during this and then when you open it, it should re update. It's only a possible fix and seems to only work for some people. It's worth a shot.
By the way, you ae backing your games up arn't you??
Not downloading them each time??