Windows 7: Having full permission, but failing to delete stuff from HDD

So the other day I had to fix a family members computer, its PSU had stoped workin, so I changed it to a PATA compatible PSU, from a SATA one, and changed our HDDs, expect my primary one that had Windows 7 installed on it. And installed Windows 7 on my secondary one, and put it in his computer. I was not able to format the disk I stole from him, so I gave the permission from trusted installed to me, but now I can't format it because "it's being used" although I cna't find anything on it that would be able to be used, and no windows open explorering it. So I tried deleting the folders in it manually, but I didn't have permission from Kiipas_computer/Kiipa (which is me) who I had given permission to.

So, anyone know a fix to this? I'd like to be able to format it, because it's faster. The things left on it is Windows (vista) and Program Files, if that would help anything.

Try cleaning the HD Drive in diskpart:
Disk - Clean and Clean All with Diskpart Command
Partition or Volume - Delete
Partition or Volume - Create New

I've tried all the tips you gave me, but I think vista is not letting me, when I tried clean and clean all it said:

Virtual Disk Service error:
Clean is not allowed on the disk containing the current boot,
system, pagefile, crashdump or hibernation volume.

The disk (called HP) with vista on it is not being used at all. Not by anything. And my primary disk which I'm running Windows 7 from is the only one being used, and able to be used.