Do you have a "recovery disk" for a specific laptop? Or do you have a Windows 7 installation disk? (Slightly different).
If you have a W7 install disk, and a product key, then you could run that. If you have an OEM W7 disk, then you'll likely load up a bunch of bloatware you don't need (or that's even relevent if it's from a different laptop/brand), and chances are you wouldn't have a valid key.