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
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers
and
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers.
Look for entries in those keys mentioning steam.exe and delete them.
Restart Steam and you're set!