you set your WEP when you set up your router, its a pain in the ass, but you need to setup your wireless network again (unless of course, you go to control pannel/Network and internet blah blah/set up or change wireless network) in windows you just need to spend about six hours trying the same thing over and over again, and then it should start working.
If nothing is working, go to control pannel/network.../network setup wizard, follow everything then make the damn wep, and try not to forget it.
I couldn't get the network up for 2 weeks because the drivers for the network card was not installed. Huge pain in the ass.