So there is a login issue going on at mojang right now with 1.6.4 and lower clients. Mojang is right now trying to fix the error... there is not ETA on the fix. You right now can only log into a server if you have 1.7 and up... Here is an offical source: http://www.minecraftforum.net/forum-151/announcement-78-bad-login-errors-on-164-and-older/
For me i no longer have this problem there are a ton of other threads about it. there is nothing you can do besides keep trying