product key usually is on the back of your laptop

Look at this article. You should be able to download a copy and install it with your product key.

http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/where-can-i-download-a-windows-7-professional-iso/785abb6a-8fa4-459e-a1b5-327b889085e5