Flip the laptop over and look at product key stamped on the sticker. Is it an OEM key? If so, you need an OEM installation disc of Windows 7. Follow the link below...
Legal Download from DigitalRiver: Windows 7 SP1, 13 languages
Edit: Before you do all of that, try going to properties and checking the windows activation pane. Hit change product key and try typing in the serial on the back of the computer.