I did not do that the first time, but I have restarted the installation again and will do that when I come across to that optionAt the very first part of windows install, what did you do with the partitions? The very first part asks you where to install windows?
Did you select custom install at the start? If not, that's not a clean install, it's just done an in place upgrade of the current installation which won't help anything.
When you select custom install, the very next bit shows you your partitions, you need to delete all partitions on the C drive and install to the spare space.