You appear to be using the correct procedure. This can be a tough problem to
zero in on. The only thing I could suggest now would be to check your system for
parasites, malware, etc. Then go to the Windows Update site and the Office
update site to verify that you have all the latest security patches installed.
It's important to have IE & OE up-to-date since some of their patches also
affect the rendering of HTML messages in Outlook.
See this page for good info and download links:
http://winsupportcenter.com/a/quickfix.htm
The above page also has a link to a site that will check your system for you.
Look for "Online Parasite Scanner".
I highly recommend Spybot S&D and Ad-aware as search & removal tools. Spybot can
also immunize your system from any future problems such as these.
For an even more proactive method of protection, see this link:
IE-SPYAD: Restricted Sites List for Internet Explorer
https://netfiles.uiuc.edu/ehowes/www/resource.htm
If I'm off-target, hopefully someone else will jump in with a solution for you.