First step would be to ping your default gateway and see what kind of response you are getting. If your cable is coming directly to your computer, you can see this ip address from an ipconfig /all. If you have a router, you will have to check the interface on the router to see what the default gateway is.
ping -t x.x.x.x (replace the X's with the IP address of your default gateway).
If you get dropped packets, then there is a problem with the ISP and you will have to give them a call to report it.
Also, if there is another computer on the local network that is doing a lot of uploading, that can cause very slow downloads. Your download speeds tank when your upload speeds are maxed out.