I'm reposting this as I've received some new inquiries about this:
According to the folks at Steam.* If you have an Error 51 it means that your ClientRegistry.blob file in the Steam folder is corrupt.* Try deleting that file and Steam should redownload the correct version of that file.* This should fix your problem.
Additionally, if you get error 51 and replacing your ClientRegistry.blob file doesn't appear to fix the problem, check your firewall software.* If the firewall software blocks some of steam's communication this can also lead to an error 51.