[He doesn't think this topic has come up with Spike. He told Kitty about it, but generally it's not something he talks about. In this case he doesn't mind sharing though.]
She was partially converted to a Cyberman. They're humanity upgraded, in these metal bodies with all emotions removed. They just convert more people - I suppose they're rather like vampires. Her conversion process wasn't competed though so it was still her, mostly and I thought I could save her. Turns out I was wrong. She killed a lot of people before Torchwood stopped her.