A guy just called me he couldn't remember his wireless password. He had computers already connected but was trying to connect another device. You could just rummage through the router settings to find it that way but Windows gives you a few options.
My favourite way for Windows 7:
Right click on the wireless connection icon in bottom right hand corner of screen. Do this on a computer that is already connected. And click on the option Open Network and Sharing Center.
On the left side click on Manage Wireless Networks.
You will be shown a list of all the network connection info saved on your computer. Click on your network name and then right-click on it and click on Properties.
In the next window that opens just click on the Security tab and check the box Show characters - you should now see the Network security key that was previously hidden by a bunch of dots/stars.
This person didn't have the Manage Wireless Networks option, turns out he had Windows 8, and Microsoft in their great wisdom decided to remove this option from Windows 8. So instead of clicking on Manage Wireless Networks, which is not an option in Windows 8, click on Change adapter settings instead.
In the window that opens find your wireless connection and right-click on it. Then click on Status.
You are now on the same properties windows mentioned previously. Click on Security tab then click on Show characters....
One good thing about Windows, is that there is always more than one way to do a particular task in Windows. I was researching on line to find out how to do this in Windows 10 since I don't have a Windows 10 setup to go by right now.
This method works in all versions of Windows. Just type ncpa.cpl in your search / run bar. (You might have to run as administrator) This automatically takes you to your network adapters, just right click your wireless, go to Status...you know the drill.