Hi,
If I run shred whole drive after a reinstall on c:/ will it delete windows and cause problems whilst doing it. Or does it just fill up the empty space and delete old windows etc?
It’s a bit sketchy in the description as it just says it will shred the whole drive, so I presume all files/folders are gone 
From the info here it seems able to erase hole drive https://www.avast.com/faq.php?article=AVKB44#idt_402
also see https://help.avast.com/en/av_free/17/privacydatashredder.html
Data Shredder leaves you drive formatted while removing all of it’s information
It does it in such a way that it becomes virtually impossible to recover any of the erased information from that drive.
That’s a simple explanation.
It isn’t fast but, it’s thorough.