After being puzzled as to why the Repair option of my custom WinXP setup never seemed to be available, I discovered this thread:
http://www.msfn.org/board/index.php?showtopic=44242&st=10
A customized unattended install disk does not support repair mode. So, go back to the original Windows XP setup disk.
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This guy has an offline tool to change to the generic Microsoft disk controllers (e.g. when you change the motherboard and forget to prep) - https://www.foolishit.com/free-tech-tools/fixide/