You can use the method below to discover the network security key on a laptop or desktop that has already set up a wifi network.
1. Open the Start Menu and click on "My Network Places".
2. Click on "Set up a wireless network for a home or small office". (We're not actually setting up a new one but you will see in a minute why we need to do this).
3. Click on "Next" when the Setup wizard opens.
4. Choose the "Set up a new wireless network". (Don't worry we're not really gonna set it up.) Then click Next.
5. When the next screen pops up their current Network name will already be in this field in the box! Make sure you have them select the "Manually assign a network key". And depending on their security settings the box beside "Use WPA encryption..." may already be checked or if they have WEP it may not be (don't worry about it). Then click Next.
6. Select "Set up a network manually". Then click Next.
7. They will see their current Network security key in bullets (hidden).
9. Now all you have to do is uncheck the box "Hide characters as I type" and VOILA! There they can now see their current Network security password.