As of about 7-10 days ago--I'm note exactly sure--when I click on a
pdf in IE 6 on any site, the pdf window comes up, but doesn't display
the pdf--all I get is a small red 'x' in  a box in the top left of the
screen. How can I solve this problem? I've read through
http://answers.google.com/answers/threadview?id=374832 and it didn't
help.

you might like to try the measures recommended here
http://www.adobe.com/support/techdocs/328233.html

The place to look for this sort of issue is in the PDF viewer and not
in the IE 6.0 browser. In the Adobe Acrobat Reader click on Edit -->
Preferences. In the "Preferences" window select the category
"Internet" and ensure that the check box next to "Disply PDF in
Browser" is checked. Check that if it is not checked already.

Restart Adobe Reader and restart the browser and you should be good to
go. If not restart the computer and try again. This should do the
trick.

The above didn't work. However, I just installed the latest version
(7) of acrobat reader and it appears to have done the trick.