reinstalling Lion to factory settings with no disks, reinstalling Lion to factory settings with no disks

Hi There, i have a problem where i have a macbook pro that has Lion installed on it. I wanted to reset it to factory default settings as i received it out the box, the problem is that it did not have any disks as the OS was pre installed on the drive. How can i reset this macbook pro with out any disks? as it won't boot past the grey screen now. Please help.