If you're connected to the router at the moment, what you will want to do is open command prompt by going to:
start=>run=>type cmd=>press enter
Then in command prompt, you would want to type "ipconfig /all" without the quotations and press enter. This will list all your network adapters along with useful network information about each adapter. Look in the section under your wireless adapter ( scrolling a bit might be necessary if a lot of information is displayed ), then you should see a property called "Default Gateway". Along the line of "Default Gateway" is your router's IP address that you're are currently connected to.