Step 1: Cleansing your Minecraft
If that doesn't work then go to "start", "Accessories" and then click "run".
When the "run" program pops up then type "%appdata%
Next step is to copy your ".minecraft" and paste it where ever you want just in case something wrong happens but i have done this 3 times and nothing bad has happened. Then after you have done that your next step is to delete your ".minecraft" and run the launcher which should update and give you a completely new .minecraft folder.