Oh s***! COMPUTER DOWNLOAD SPEED DIING!!!!?

More than likely it has nothing to do with your computer. Internet Service companies will automatically slow your connection down if they see you are using a lot of bandwidth. This is so you don't slow down the other people in your neighborhood with your mive downloading. They often set a cap on megabytes per hour that you are allowed. So, in an hour, it'll speed up again until you exceed the cap again. Just leave it overnight and it'll probably be done in the morning.