Malwarebytes had a bad signature which flag many core windows files as malware, then quarantined them. It also quarantined itself rendering many machines unusable.
See mkleczynski's comment at <a href="http://www.reddit.com/r/computertechs/comments/1cgk82/malwarebytes_database_version_v2013041512_flags/c9gfvy8" rel="nofollow">http://www.reddit.com/r/computertechs/comments/1cgk82/malwar...</a>