Please try deleting the 1.6.4 folder ([minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/versions/1.6.4) and then redownloading 1.6.4. Does that fix your problem?
kumasasa
Are you sure you can set -Xmx1300M on your computer (32bit OS) ?
Please try deleting the 1.6.4 folder (
[minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/versions/1.6.4
) and then redownloading 1.6.4. Does that fix your problem?