I get that occasionally myself, and haven't figured out why. Rebooting has
fixed it each time it's happened to me.
The first time it happened, I panicked and used the Control Panel Programs
applet to repair Office 2007 -- and that worked without rebooting. But,
repair takes a long time. The next time it happened, I just rebooted (which
took less time than doing a repair), and that seemed to fix it as well.
My assumption is that some program was stepping on the memory image of
certain registry settings, and repairing or rebooting causes those settings
to be refreshed, solving the problem.
I wish I knew which program causes it... but I haven't discovered it yet.
