Assuming you are not in the US, you need to contact support in your region or obtain a Dell installation disc from someone/someplace.  You can also download a "retail" ISO and activate by phone with Microsoft using the product key on the COA sticker. www.w7forums.com/official-windows-7-sp1-iso-image-downloads-t12325.html
Hello: Recently I brought DELL laptop - Inspiron 15R 5520 and this comes with preloaded Windows7 Home Premium. I want to format my laptop and do partitions to my hard disk. Please help me out
How to format my DELL Inspiron15R