Removing the .minecraft folder in %appdata% will remove Minecraft completely but you will also lose you save file. Backup the saves folder in there first.
A mod that crashs your Minecraft? Just click force update in the MC launcher. You change only the minecraft.jar with the mod thats why you don't need to remove the whole folder.
If you want to deinstall in 100%, hit start, and in the search bar, type %APPDATA% with the %. Then, move into roaming, and there is your .minecraft folder. Just remove this folder and you're done. But it is better to revert your minecraft by hitting force update when starting the game. This is found under options, near the login button.