XP Setup lost CDRom

I was working on a PC that had developed a problem with the CDRom (DVDR actually). The drive was no longer listed in "My Computer" and had an exclamation mark in Device Manager.

I uninstalled, restarted, etc several times then decided it might be easier to reinstall XPH over the existing installation, i.e. installation repair.

Booted from the CD all ok and get throught the "DOS" installation part fine but when it reboots to the standard XP installation screen the CDRom is not found and the installation just halts at

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The file 'asms' on Windows ...... etc is needed

Copy files from GLOBALROOT\DEVICE\CDROMO\I386
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Now I can't go forward because there is no CDR to continue the installation with or back because system restore won't work mid "Setup ".

I've tried copying the XP files to a second drive (which can be seen and explored) but when the setup halts there's no browse button and typing any path has no impact.

Any suggestions to get forward or back would be greatly appreciated.