On your computer go to the START menu, then RUN, then in the RUN box type in CMD..then at the command prompt type in IPCONFIG and you will see the default gateway IP...If it is a default setup it will most likely be 192.168.1.1 or 192.168.0.1
The default gateway on your PC should be the PRIVATE IP address of your router... As win2k mentioned above, you can see it by performing the IPCONFIG command.
Your router's manual will state what it's default IP address is. You can reset it to 'factory defaults' (erases existing config) if you push the 'reset' button on the back with a pin or paperclip.