I've encountered this from time to time, usually within the VBA
editor, not the Excel UI. Usually switching to another window and back
cure the problem.
You can install a free program called FreeWheel. This restores
wheel-scroll operations when they "disappear" and can add wheel
support for controls that don't ordinarily have such support. I've
used FreeWheel for probably 10 years now and have never had a single
problem with it. Not one single problem over the course of using it
under Windows95, 98, ME, 2000, XP, Vista, and Windows7. To say that
it is stable is an understatement.
You can download the latest version (which is some years old but still
works like a charm) at
http://www.geocities.com/SiliconValley/2060/index.html
Cordially,
Chip Pearson
Microsoft Most Valuable Professional
Excel Product Group
Pearson Software Consulting, LLC
www.cpearson.com
(email on web site)