There is not a Dell version of Windows 7 to purchase.  Dell Windows 7 media differs from Microsoft media ONLY in that it activates automatically when installed on a qualifying Dell computer, and it has some branding items (background/user images, support logo, etc.). A "retail" key and included media will be fine. You can find Windows 7x64 drivers for the 660 here:[URL] 
After some months of sporadically trying to access a Microsoft Account or the Store on my Dell Inspiron 660 without success (along with some 1000's of other Win8 users according to M.S. blogs), plus its incompatibility with *all* of my USB external audio interfaces in spite of driver updates, the time has come to downgrade to Win7 from the preinstalled Win8 Home.  I am assuming the software is the problem as the hardware works fine otherwise and everything will work in Win7. Thus, the question is will any commercial version of Win 7,  eg. from Amazon, be 100% compatible with this machine, or are there some tricks with the bios that would need fixing.  Is there is a Dell configured version of Win7 to purchase.   
downgrade Win8 home to Win 7