I want to install Zorin in my existing Windows PC. I don't want duel boot. I want to install Zorin in my existing C Drive so that the windows will be erased but the other drives will remain intact.
You should be able to delete the C drive from windows disk management and install Zorin on that unallocated space using the something else option on the installer. If you do this make sure you are ready to go first , backup , create your Zorin boot medium, and set the bios to boot from usb or dvd. Can you post a screen shot of your partition layout in windows ?
Edit : Along with the C drive there will be a system reserved and a boot related partition that will serve no purpose without windows. You may have use gparted from the live boot medium to delete and format the C partition to ext4 because Windows may not let you delete the entire volume while mounted. It is possible to shrink and delete most of that volume from windows disk management.
If you are using Windows 10 then you should be using WSL so it runs as a VM - I don't use Windows and WSL had big security issues - not sure if they were resolved in WSL2!
However, Zorin sadly, isn't listed. Your best bet is to try Virtual Box for Windows and run it inside of that but it will not compare to a hard metal install.