Perhaps I didn't make clear that I'm using SP1, not SP2, in my normal operation. So the rollback wasn't from SP2 to SP1.
Another source suggested that my use of the external HD may be confusing XP. It may expect the external HD to be there on bootup, but I only turn on the external HD when I use it for backup.
I'm not 100% sure, but I think I used the external HD the day just preceeding the latest "problem" startup message. I had shut down the HD a long time before turning off the computer, however. This scenario is likely to have occured on the earlier startup "problem".....
I hope this analysis is correct.
Thanks for your comments,
sc