A spreadsheet at work has begun to throw up an error message saying

"Cannot Clear Clipboard"

when attempting to copy cells. It seems to happen after about 100 cell
entries have been made.

Anyone know what could cause this?

The spreadsheet is very simple - really just a table, no formulas. The
only slightly unusual features are two columns which mostly stay
resized to zero width, and the fact that the workbook comprises about
25 worksheets.

This is Excel 97 SR-2 on XP.