^
Basically what he said. Removing the .jar fixes nearly everything, and if it doesn't work, I'll investigate further and let you know what I come up with.
How to remove and reinstall minecraft~
Go to the minecraft.jar in your minecraft folder, (assuming you know how to get there) and drag it out into the trash. (Be sure to save any important minecraft worlds by putting the folder containing them on your desktop.)
Now that the minecraft.jar is gone, go to your launcher, and on the login screen, click "options."
Select "Force Update", fill your in your minecraft login info, and click "login."
From there it'll replace the minecraft.jar with a new one, and more than likely fix your problem.
Hope this helped