I had the exact same issue, and tried deleting and installing and all that, which didn't work for me either
I went to the Minecraft.net site and clicked 'download minecraft.exe'. Select "Run" rather than 'download'.
It opens a new window called "Minecraft Launcher" that shows all the versions, including Beta 1.8.
In the lower right corner it asks for your username and password, which you put in. Next to your username is a button, which you click on (it probably says 'update' but mine was blank).
When you click on it you get a little box called "Launcher Options" that has a button labelled: "FORCE UPDATE", which you select, and it will go ahead and update the thing for you. After that it should launch, and you'll get a cool new start screen.
PS the new mod is pretty awesome - enjoy!!! :nD
Mike