I am using Excel 2003 under Windows XP Professional SP3 (as I recall it).

I am a writer, and I use a spreadsheet to sort material for books, using
three columns, ch, pt and no, (chapter part and number). Every so often, out
of the blue, Excel forgets the standard three-part search and defaults to not
using a header row. Once or twice it has come back, but not usually.

Sometimes, I can paste all of the data into a fresh worksheet and get the
search running again, but sooner or later, it breaks down again.

Can anybody tell me what I must have done to cause this, or better still,
how to fix it?

If my methodology sounds odd, keep in mind that I typically write historical
stuff and usually have a dozen columns, and 2000+ rows. At times, I may sort
by date, location, accession number or other headings, and using the sort
function as I do allows me to insert extra data, quotes and other stuff--and
get my original order back again. And I'm getting too set in my ways to
change, OK?