Connect the computer to the router by ethernet cable. Power the router and restart the computer.
In Windows go Start, All Programs, Accessories, Command Prompt. At the prompt type IPCONFIG /ALL
In the results you'll see the word gateway and beside it should be an IP address probably in the range 192.168.0.1 or similar.
Failing this locate the literature which came with the router and download the full manual which should mention the default IP address -- if you think it may have been changed restore default by pressing reset.