Dual hard drive installation

I am new to this community.  I have built 4 desktops and am now building a new one.  I want to install Windows 7 on 1 drive and install a second one as a secondary drive.  Both are used and one has a version of Windows 7 on it.  Should I install them both before installing Windows 7 or should I put one in, install Windows 7 and then add the other one later?  My concern is that the second drive will have a copy of Windows on it.  I am not sure how the disks will format.

Hi yes you don’t want to have 2 different copy’s of windows on the same pc. When you install a program you had before it may use the old windows version. When installing hard drives it is best to only have one drive plugged in for the windows install then add the  other one after. I slipped up once and did not know the second drive had a copy of windows on it and a few programs when I installed them used files from the other windows, delete the windows folder first or format that drive. You can get a boot disk that will format the old drive first.