If the laptop came with Windows 7, then click the following link to download a Windows 7 ISO. You will need to enter your Windows 7 product code which should be on the underside of the laptop. Burn the ISO image to a DVD or copy it to a USB drive to install.
http
/www.microsoft.com/en-us/software-recovery
If your laptop came installed with Windows 8 (8.1), then click the following link. You will need a USB drive or a DVD burner. The program will not allow you to save the Windows 8 ISO image to your hard drive. No need to worry about entering a product code because it is embedded into the laptop's BIOS which will likely be verified during the installation process...