Blank windows

I have been trying to solve a problem on my wife's computer that gives us
blank windows in both Outlook and OutlookExpress when we try to read or reply
to a message. The problem seems to have started when we switched from Norton
to MacAfee.

I have fixed twice but it comes back. We are using XP Pro and have SP 3
installed.

Full scans with MacAfee and the Malicious Software Removal Tool find nothing.

I tryied to re-register some dll files. Most registered OK, but for
mshtml.dll I received the message that the file was loaded but was not
registered because the entry point was not found.

I searched my C: drive and found more than 40 copies of this file in various
folders, many of them that seem to be related to IE upgrades or NTUninstall.
The C:\WINDOWS\System32 folder contains a 3,511 KB version of mshtml.dll with
a last modified date of 16/01/2009.

Suggestions would be welcome.