You can download Windows 7 Ultimate ISO image from the following locations. Once downloaded, burn it to a DVD and install using the product key found in the certificate of authneticity (COA) label present inside the battery bay (you can see the label once you remove the battery).

x86/32-bit with SP1:

http://msft.digitalrivercontent.net/win/X17-59463.iso

x64/64-bit with SP1:

http://msft.digitalrivercontent.net/win/X17-59465.iso

See here for additional instructions including applying the Dell OEM System Locked Preinstallation mechanism:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/windows-7-sp1-iso-download/

See A Clean Install of Windows 7 for installation instructions:

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows/a-clean-install-of-windows-7/

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