BootMagic would work, but I would probably do this in the BIOS if I were him.

Many new motherboards with AMI bios will allow you to hit F8 at POST to select what drive to boot off of. Since the Promise controller is seen as a SCSI card, just hit F8 and tell it to boot off the Promise. Win98, I would assume, should not have a problem booting fine from the Promise controller even though other physical hard drives are present on the Pri/Sec IDE controller.

If that didn't work, I would imagine you could work some magic playing with boot.ini on the Pri Master partition. But, at that point, bootmagic is no-hassle and works great (the last time I used it).

Anyway, I don't think DiGNAN will be too worried about dual boot now. I've been talking with him through IM about this and it seems his D: drive is gone. I am not a data recovery wizard, so I was unable to give much advice.