If you want to install Win 7 or 10 then just choose custom install and delete all existing partitions (clean install).  Win 10 Pro is better because it is internet safe and easier for drivers install.  Use the Win 7 pro key to activate 10. It works.
I purchased Windows 10 and tried to install it, but it won't install. It gets to 73% through the first blue installation window (before any restarts) and then stops and I get an error message that says "Windows 10 installation has failed." Nothing more specific. I Googled the problem and tried all kinds of fixes, to no avail. Any suggestions? Currently running Windows 7 64-bit Pro. Intel Core i 7-6700 CPU, 16 GB RAM. OptiPlex 7050.
Can't upgrade from Windows 7 to Windows 10