|The Dell product key won't work with the non-dell install disk as you discovered.|
When you formatted the harddrive in another machine you removed any old bootfiles from it and that is why you are missing the bootmanager when you try to start up now.
You need to go into BIOS and set CD as first boot device.
That still doesn't resolve your product key dilemma.
you may be able to obtain a Dell install disk from ebay
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