Plug your computer directly into the modem. If you get close to 20mb then yes, look at your router. If you still get under 10mb then it's not your router.
However do this in the afternoon, early morning as peak times wll throw your speeds off.
You can check to see if you are on 20mb by checking your rate @ click here [this should open at your modems homepage].
p.s. you do have a 100mb or 1000mb [1gbps] LAN connection [look at your network card for supported speeds]? If it's a 10mbps card [and an old one at that] then this will also cause you a problem.