Problem resolved - following a suggestion, changed the drive setting in the Bios to ATA. Then the XP reinstall disk fired up and the repair worked instantly.
I am having trouble repairing XP on an Optiplex 755. The Windows config (I think) has become corrupted, so it will not boot into Windows but says run R on the O/S reinstall disk. The trouble is that the machine was supplied as Vista/XP downgrade, and the reinstall disk goes straight to Vista and cannot find the XP installation to repair (although I can get to a cmd prompt that way). I have tried using an older XP disk, but that fails, I suspect because it doesn't have the right drivers included. Help please!
Unable to repair XP on Optiplex 755