[rockfan4]

Ensure you have a good backup of the drive
Try it from the command line.
Start - Run - cmd
run Diskpart
List disk
Select disk (probably going to be 1)
list partition
select partition (whatever partition number it happens to be)
Delete partition
If that doesn't work, and you're really sure you have the right partition selected, Delete partition override
Repeat as required.
Exit
If you delete stuff you wanted, it's not my fault.
Restore from backup if you screwed up.

Or download and create a gparted live usb drive, boot from that, use gparted. Better