check your pc's case, disk, manual and website. if the os came bundled with the pc, it should have the serial number printed somewhere on itself. you can also try contacting dell customer support.
if you want to find out your own pc's installed os license number, try magical jelly bean key finder tool http://www.magicaljellybean.com/keyfinder/
you can buy win 7 license from microsoft and get a license key from them.
you can buy an oem key for cheap. it'd be tied to your pc's hardware.
you can also start using a free os like one of the linux distros e.g. mint, ubuntu, mandriva etc.
you won't find product keys here.
tomshardware does not support piracy.