wireless internet trouble

I had a Windows XP computer in my daughter's room receiving wireless internet just fine. She received new HP pc with Windows 7 and now it recognizes the wireless network but says the correct password key is invalid. I am running it with an Apple airport extreme router, any ideas on how to get it to work. I have never had a new machine or anyone's laptop not launch right away? Thanks for your help.

If you open up the router via a web page and check the passcode for the network to verify.

Or change the passcode all together, assuming there's not too many machines that will require the change to give you a lesser headache