Your product key is affixed to computers purchased from OEMs with Windows preinstalled. It is also provided with license agreements and may be accessible from Microsoft Volume Licensing Services. Product keys are also provided with media — usually on the back of the CD/DVD sleeve.
If you do not have it any of those places you do have the choice to call support. You can select “register off line” since not all people in the world do not have internet access believe it or not. However they are going to ask for some validation, UPS from box or the like. Bottom line is if you did not personally purchase this OS and cannot provide proof of purchase you aren’t going to get it validated.
Also, you can search on case of WIndows XP CD. Especially on the leaf which comes in case. Also, sometimes branded machine stick the license on machine somewhere on tower or on laptop bottom