if you want to find out the serial number for your own windows 7, you can try magical jelly bean keyfinder tool. http://www.magicaljellybean.com/
if you're asking for product keys of windows 7 os that you do not own, that is piracy and we do not support piracy here at tomshardware. it is against the rules.
buy a windows 7 yourself and you'll get a product key. you can save money by buying an oem disk (tied to one hardware configuration).
alternatively, you can use one of the free linux distros - no hassle with money.