If you purchased the game through the indie bundle/gala. Please register the game on your forum using the Steam key. That should work.
If you get a small file from Steam and can't start the game. Please try uninstalling it by "removing all local content". Also delete anything related to KA2 from your user and steam libraries manually. Then restart Steam and try installing again.
If you get a small file from Steam and can't start the game. Please try uninstalling it by "removing all local content". Also delete anything related to KA2 from your user and steam libraries manually. Then restart Steam and try installing again.