This worked for me.. skyrim at least..
http://forums.steampowered.com/forums/showthread.php?t=2459905
If you can not start your games.... something i did.
Start steam
go to settings (downloads + cloud)
untick "Enable steam cloud synchronization for games which support it"
shut down steam and wait for the exit now to clickable and click it.
open steam
Go explorer and go to your steam folder
Rename the folder "appcache" to what ever you want
shut down steam
and restart steam
Wait for the pictures of your games load. Could take 1 or more mins.
**AGAIN** untick "Enable steam cloud synchronization for games which support it"
Then you can play games.
http://forums.steampowered.com/forums/showthread.php?t=2459905