You can upgrade your own PC using a Windows 7 license key. If you perform an in place upgrade following this guide here, it should not even ask you for the key. However you can use Magic Jelly Bean Keyfinder to recover you windows 7 key if it is not printed on a sticker attached to you laptop or desktop. Please note sometimes the key was placed under the battery in a laptop if it was removable.
Please note that Magic Jellybean Key finder will be flagged as a virus / malware so you will have to disable your antivirus temporarily so it does not automatically delete the program.
Let us know how it goes!