I have heard some people mention that you cannot use the original DVD to recover once "Service Pack 1" is installed but I never tried it for myself. So it is plausible that this is your issue; you may have upgraded to W7 SP1 (part of Windows automatic updates) and the disks were created when you were just W7.
If you want to download and create a W7 SP1 DVD you can find links
HERE. Be sure to burn it to DVD with a product that understands what an ISO image is (example
IMGBurn in our
Freeware DB). Please note this is not the same as an OEM version you would get from Dell or Sony, this is the same as the retail version you would buy in the store. Be sure to get the same version for which you have a license key and also get the same bit size (x86 for 32-bit or x64 for 64-bit).