C for Windows (yep, I can see the logic in hiding this elsewhere from hackers, but there it is)
Oh yes, Once Upon A Time I was running NT Server from D:, and I had 98 installed on C: in case I wanted to play any games; 99.9% of the time I was actually running NT Server from D:.
Somehow a virus got itself onto my network. It very cleverly installed itself in C:, where I spotted it long before I next intended to boot from that disk. Not the first time that a bug in a virus has rendered it harmless.