How i can get the os with old key number in dell

Not exactly sure what you are looking for, but think it is as follows, If you have a license sticker attached to the notebook, you can install the OS using the product key on the sticker, you will need to get a physical copy of the OS on disk though to do this, ask around friends, work and family to see if anyone has the correct disk for your license, or look on e-bay for installation disk (no need to repurchase licence if you have license sticker attached)