If you mean literally everything (including the Windows folder), then you need to connect the drive to a second computer so it doesn't boot from it, then simply copy and paste the files from one to the other. If you want to leave the system files and just transfer everything else, you have to do it folder by folder.
A little edit: Alternatively, if you have a USB drive you can boot from (one with Windows or an OS installed on it), boot using that so the permission ban to move around the system files from your main drive is lifted, then copy paste the contents of C to the new drive.