Is it one particular workbook or any?
If it is a single book then I suspect the 'normal' style is becoming corrupt
and I would copy everything out to a new book, after taking a few copies.
If it is Excel generally, I would try these things in order, closing and
starting Excel each time.
1) Uncheck all add-ins from Tools>Add-ins and if this stops it then add back
one at a time until the issue returns
2) Take your *.xlb file (toolbar customisation, it may be hidden) and move
it from it's location. Excel will create a new one
3) Remove any files from your xlStart folder (personal.xls, book.xlt,
sheet.xlt, etc) and if this fixes it, place back one at a time to see when
the issue returns.
4) Run detect and repair under the help menu
--
HTH
Nick Hodge
Microsoft MVP - Excel
Southampton, England
www.nickhodge.co.uk
(e-mail address removed)