You log in to the router via your browser. Check the router's address by going to Command Prompt (Start, All Progams, Accessories, Command Prompt) and typing IPCONFIG /ALL the results will show the Gateway IP address.
Put this IP into the address bar of your browser and you should be greeted by a login box -- User name is always admin and the default password is usually admin or password or leave it blank. If the default has been changed you'll need to reset the router using the reset button behind.
For heaven's sake download the manual and read it ! And do follow the advice to disconnect the router from the modem.