I pressed the install button and a message appeared telling me that it was updating my files. However, the updating continues forever.
If the game is installing rather than simply patching, the download may take a LONG time.
If you're sure the game is already installed, try this:
Right-click on the item in the Steam game list and select properties. On the updates tab select "Do not automatically update this game".
You may want to check the game folder and see how large it is.