if you have the case of the disk you can contact with Microsoft and say that you lose your cd and say the s/n of disk ( it's behind of cd case )...if all that you say it's truth. then it should be some sticker behind your pc...
When you power on the computer, hit Ctrl F11 after you see the Dell BIOS splash screen. It will boot from the Dell System Recovery partition and you can reset Windows to the way it shipped from Dell.
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