I thought this was no longer the case with the v5 installer. Well i guess i need to do some testing next time i have access to a windows machine.
<rant> Who the fuck thought it is a good idea to set KiCad specific variables at the operating system level? And why would only the windows installer do something like this? Windows of all systems where you would not expect users to have deep knowledge about such things. Where you can not even rely on the fact that every version of it even exposes a user interface to change this stuff. </rant>
@LHC i sadly can not really offer a solution right now. I have no way to test if a solution i find on the internet works.
You could check if the variables are defined in the tool shown here: https://www.digitalcitizen.life/how-edit-or-delete-environment-variables-windows-7-windows-8
If they are then you hopefully can remove them using said tool.