
Windows
Navigate to your Application Data Folder: %AppData%
Double Click the .minecraft folder
Copy out the saves folder to a different location
Go back one directory (to where you can see .minecraft)
Drag the .minecraft folder into the trash
Start the Minecraft.exe file to re-download Minecraft
Close Minecraft when you reach the title screen
Copy the saves folder back into the new .minecraft folder. You can safely overwrite any data that's currently in this folder