iMac is stuck on blank grey background

Hello everybody,

2 days ago I bought an Imac 2011 for round about 20$, the man Selling it told me that his mother used it for basic stuff and told me that some Day It stoppen booting, he removed the harddrive and gave it to me, I replaced it with an old hdd I had and after starting the Mac it booted to Windows 10 right away, no Problems Expect the missing drivers for the gpu and Audio, while trying to install Mac OS high sierra it always stopped after 70% on the Apple logo and after a while the Apple logo disappeared with just the grey background nothing else. Well I can install windows with no Problems but cant get past the Apple logo. What can I do?

Also Im trying to install windows 10 right now just to test if everything works fine.

Edit: installing it went fine, but its stuck on the windows boot screen, was Not like this before.