I would start by connecting one machine directly to your cable/dsl modem and rebooting the modem. Then go to speedtest.net and check your speeds. If it comes back close to what you speed is rated at, then at least you've got it narrowed down to a router, or one of the machines on your network hogging all the bandwidth. If it comes back still very slow when connected directly to your modem, call your isp and haven't them come check things.
If they are shaping traffic which they almost certainly are, your speedtest results shouldn't be affected, but in your case that came back slow as well so it's likely something else.