While connected to your router, do an ipconfig command in the command prompt window (cmd.exe) and you should get your default gateway, which you can then use to connect to the router's config page. It's usually, like Scourge said, 192.168.0.1 / 192.168.1.1 / 192.168.100.1 / similar.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Users\JP>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Wireless LAN adapter Wireless Network Connection:
Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::58f5:6eb3:7e70:9465%11
IPv4 Address. . . . . . . . . . . : 192.168.1.137
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1
Tunnel adapter isatap.{C6539F9A-2515-4635-AF67-655B5F7C069D}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Tunnel adapter Teredo Tunneling Pseudo-Interface:
Connection-specific DNS Suffix . :
IPv6 Address. . . . . . . . . . . : 2001:0:9d38:6ab8:2cd3:f5ac:47cb:e6f1
Link-local IPv6 Address . . . . . : fe80::2cd3:f5ac:47cb:e6f1%14
Default Gateway . . . . . . . . . : ::
Tunnel adapter isatap.{92DCA394-D76E-44AB-85D2-3492D1C7DA80}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
C:\Users\JP>