Association between cells

I am using Excel 2003 on Vista.
I have a .xls provided to me.
On one sheet, cells(21,1), cells(21,2) and cells(21,3) have values.
cells(21,3) is a number.
When I change [21,3] to a value which is not a number,
the texts in [21,1] and [21,2] are changed in color from black to red
and they are crossed through with a line.
Writing "1.2 abcdef gh" to [21,3] has no effect; "1.2 abcdef ghi"
changes color, etc.

Can somebody suggest mechanisms which might have this result?
A dump tool to turn a .xls to a .txt might show the actual mechanism.
Can somebody please point to one?

As it happens, I can work around by writing [21,4] instead.
I am unhappy to find something I don't understand in the sheet.

No cells have validation - shown by edit/Go to/special/data
validation/all.

Ah ha! I've found it. [21,1] and [21,2] have conditional formatting.
[21,1] and [21,2] have Condition 1 as Formula Is =$C21>5 with a format
of strikethrough on a red background.

Selection.FormatConditions.Delete eliminates the conditional formatting.

--
Walter Briscoe