When I said about not going into detail, your suggestion was part of what I meant (not that you could know that).
As the title suggests, I've done every check I can think of and a few others. I've sfc'd, I've re-installed windows, virus checked, trojan checked, turned somersaults etc etc.
My feeling is (and Microsoft reported this with Office XP) that there is some form of interaction between Symantec and Word being used as the email text editor in Outlook/Outlook Express.
It was for this reason I switched to using Eudora as my email client. The problem seemed to fade away, but recently (lured by the fact that my wife's Centrino runs the Symantec and the above setup without complaint) I re-installed Outlook 2003 and Hey Presto, problem recurs.
I've now gone back to Eudora as my principle email client and have noticed that the incidences seem to be lessening. It's a pity because Outlook is such a GOOD piece of software and I use it a lot.
I've got rid of Symantec antivirus/antihack programs and now only have Norton Ghost on my computer (System Restore can then be turned off).
I was seeking a 'straw poll' to see if others were similarly afflicted.