For Windows:
-Obtain a clean copy of 1.7.3.
-Go to Start, and search for %appdata%.
-Then go to Roaming / minecraft / bin
-Delete the minecraft.jar in this folder, and replace it with the one you downloaded.
For Mac:
-Obtain a clean copy of 1.7.3.
-Go to Library / Application Support / minecraft / bin
-Delete the minecraft.jar in this folder, and replace it with the one you downloaded.
Hope this helps, and if I were you I would back up the entire minecraft folder every time a major update is released, which is every time the second number in the version changes (the x in 1.x). Then let the game recreate a new folder, and if you want any of your old saves, drag those over to the new folder.
-MastaC