Check you WAN settings, make sure the connection between router and modem is set to Auto. I set it to 100 and 1000 and saw a decrease in throughput.
Now, I am assuming you are checking internet speed via hard wired connections. If you have your wireless connection improperly setup, you might suffer speed loss.
It sounds like you computers are setup correctly, but, try TCPOptimizer from www.speedguide.net. I know it works on XP and Vista, unknown if it would help W7.