The correct way to delete an OS is to delete its partition in Disk management after deactivating it, using Diskpart
Delete Partition Override command, or using a Partition Manager like free Partition Wizard bootable CD.
However if you have a lot of data on the partition which cannot be easily transferred out then sometimes deleting the Windows folder is done - but it is never easy and is like pulling teeth.
First gain ownership using this:
Take Ownership Shortcut
Folders which won't Delete have to be stripped out from the lowliest file by rightclicking Security tab to full permission Everyone.