I have an Optiplex 755 used as a second PC that has been successfully running Win 7-Pro for several years.  Following a partial SATA HD failure, I decided to replace the failing HD with a new and larger SATA HD.  When I went to install Win 7 Pro on the brand new Western Digital 1 Tb SATA drive using my original Microsoft installation media, the installation started normally and then stopped, calling for a SATA driver.  I downloaded what I believe is the correct SATA driver, R173415,  from Dell Support.  The Win 7 Pro install process refuses to recognize this driver, claiming it is 'unsigned'. 

What is causing this problem?  I'm in my mid-80s and although I go back to the MS-DOS days with my old IBM PC, I am way out of my depth with this problem and don't know what to do.  HELP!