Are you using wireless access or wired?
If you are using wireless, you should first see if the problem is a wireless problem or not. You can do that by running an ethernet cable directly from your PC to the router. If that gives you full speed, then you know your connection and computer are fine, it's a wireless problem. If that's the case, I can't help you, but at least you will have more information.
But if you still have slow throughput with a cable, it's not a wireless problem, obviously. How far are you from your provider's central office (CO)? That is, how far does the cable run from your house to the phone company? Back when I was looking into DSL, Verizon confessed that I would only get half speed because I was so far from the CO (but I would still have to pay full price).