Windows disappear/vanish/become invisible when working in Word or Excel (MSO2004)

[doyouflip]

When working in Word the active window frequently becomes invisible. I
can still type and use keyboard commands. If I do any of the following
the window reappears:
- cmd+tab off Word, then back on
- save the document
- select Window > document name from the menu bar

When working in Excel I have the same problem with the same solutions.

In both cases, this happens so frequently that both products are
essentially unusable. Any ideas?

Thanks