PowerPoint Viewer 2007 won't display equations

[Eric]

I have some PowerPoint files which include equations created with the
Microsoft Equation Editor. They look fine in PowerPoint. But when I try to
view them with PowerPoint Viewer 2007, they're displayed as a frame with a
little red X in the upper left corner.

I have verified that the MS Equation Editor is installed on this computer.
(I've checked this by looking in Word 2003 and selecting by Insert/Object,
and Microsoft Equation is visible as an option.)

Any suggestions?

Thanks.