Many are configured to DHCP an address to your computer, if you get an address from the unit, then you can simply check your Network Settings and use the "GATEWAY" address as the units address.. to check go to a run line and type cmd then on the "black screen" that appears just type, ipconfig /all be sure to leave a space between ipconfig and the slash. You can read the gateway. I think MOST of them will redirect meaning you can try http://speedstream to get to the configuration page. If that does not work, try http://192.168.254.254 (the factory default) or http://10.0.0.1 which is a common default. However many ISP's changed that in the firmware! For instance Big Pond used 10.0.0.138 for the address.
Here is the manual
http://internet.bell.ca/img_gallery/SpeedStream4200_EN.pdf
or from windstream
http://www2.windstream.net/alltel/portal/customersupport/?hrefURL=/dslhelp/checkupCenter.html
and an article on how to fully reset one
http://atlantatechpro.com/howtos/howtosnetworking/speedstream4200/78-factoryreset4200
and use the interface.
There is also a good info page at
http://bc.whirlpool.net.au/bc/hardware/?action=h_view&model_id=278