I have a problem with my os on my hard drive so im getting a new os would i be able to download the new os on the same harddrive or can i download it to a usb external drive or do i need another sata hard drive
The download is just a DVD image file, so it all depends how much free space your existing drive has. If there's enough free space for the download, go ahead. When the download has finished, create an installation DVD from it. You can then boot from the DVD and delete existing OS partition before installing new copy of Windows.