Hi - thanks for that. I've followed that link, and done everything it said. The good news is that it worked in that what it said would happen DID happen.
I installed my XP disc and copied the I386 folder across. The SFC Scannow then completed without further problem.
However, I have now rebooted the computer and my initial problems remain. I've yet to do what DennisD suggested and do it in safe mode, but the fact remains that DAF is unable to complete the process - and I keep getting the same error message.
And I still can't access Search Companion, Help and Support, System Restore or a number of web pages!