If your network security key is not printed anywhere you could try this
open Control Panel > Network and sharing >View Network and sharing tasks, on that page you'll see your computer name, your internet service provider name and a globe. beneath that there is the name of your ISP again and Wireless connection, click on Wireless Connection, another window will open in that box, properties, click wireless properties, another window will open with two tabs, click on security, tick box to show characters write those characters down somewhere and that's the security key you are looking for (It's easy when you do it once). the characters wil be hidden by default next time you try that - if ever you have to do it again, but don't leave this security key left hanging around as someone could use your WiFi without your permission