Today I had Steam start throwing an error that it is being run in compatibility mode when I had never set such a thing, nor were either the shortcut or EXE itself flagged for compatibility mode. Thanks to a little googling, I found this thread on the Steam forums which finally had a solution.
Open the registry editor (if you don't know how to do this, you should
not be messing around in the registry) and browse to
Look for entries in those keys mentioning
steam.exe and delete them.
Restart Steam and you're set!