I have a different modem. I used to have my PC plugged into the modem and no router.
When i changed to a router i powered down the modem, powered it up with nothing connected, plugged the router into the modem and power that up, then finally powered up the PC.
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when you reboot the PC does it all work fine for a while and then suddenly slow down?
Have a look at solution 1 on this page. I had to do this after i installed DD-WRT
http://www.dd-wrt.com/wiki/index.php/Router_Slowdown