If you have Retail Installation Media. Make a Bootable USB and delete the EI.cfg file in the sources folder. Install Windows 7 Pro without a Product Key and then apply Dell OEM SLP. [URL] 
Hello, I have Dell T5500 workstation with windows 7 pro 64 bit. There is a win product key sticker on the casing. However Dell windows installation cd is lost. Is there any way I can download iso image again or retrieve this cd/dvd? I have tried to use this product key with another windows installatiot but key is not recognized. Microsoft support sugfested me to try to obtain oem install disk again. How to do that? Thank you!
Retrive Dell Windows installation disk