The only options the blank partition gave was 'New' when right clicking, then it complained that there were already a maximum of four partitions, and I couldn't have any more. It was fine, as they'd managed to save all they wanted anyway, so deleting D wasn't a problem.
I then had another problem which turned out to be a dead router, and now it appears to be full of viruses, so I'm going to reinstall Windows, which it would have been easier to do in the first place. After that, I never want to see the sodding machine again.