I will tell you how to fix the depressingly and frustatingly problem that caused many people who used Steam. I found the solution for this just a several minutes ago and likely to post it there. Okay let's go to the step...

1. Open up your Steam application.
2. Right-click the game that have a problem of Error 51, then click Properties.
3. Left-click tab LOCAL FILES, then left-click VERIFY INTEGRITY OF GAME CACHE... .
4. Wait until process completed, after that Steam will detect some file that missed up and reacquire it for you.
5. If the download done, start the game.
6. Done and finish!

That's up! I will post any useful information if I have time and got it, of course :P. Good luck and best Regards from me.

Source : geniousplus.blogspot.com