Might not be related to you but I had a similar issue due to a bad disk. Replacing the disk fixed the issue. God I hate windows; if linux I would spotted the issue right away.
I've been having issues lately with Steam specifically playing Counter Strike GO. Many times when I'd enetr the game it would tell me some files were missing signatures and I would not be able to play in VAC secure servers.
Some googling told me to "verify game cache" which seems to work when this happened (probably 10 times over the course of a few months). Well it has now happened again except the typical fix will not work. I uninstalled the game, cleared my download/temp folder and reinstalled. Tells me CS needs an update.. Okay so I download the update and what do you know..? The DL fails because the "update has a corrupt file". Tried uninstall and repeat this process a few times with no luck. I'm getting really pissed because I am a part of a community with online friends and people I chat with daily, and now we can't play. I know Steam will be useless with tech support...ugh.