I know it's a pain, but I know I don't have a lot on my machining PC, just dos, and my cnc files which are backed up on my main comp. It may be worth while formatting the drive and re-installing dos. Obviously not a choice if you're running windows on it as well, but a thought. Personally, I'm using freedos, which can be found
over here which is free, and you don't need windows to run it. I've not had any problems with it at all.