I did a google search on it, and found someone who said that it could be caused by a trojan virus. Have you run a virus scan lately?
Someone else said that this is supposedly a known problem in IE6, and is fixed in a later hotfix. Next question, then, is your system updated? They were kind enough to provide a microsoft link. I know it says problems when you open email message, but at the bottom of the page it says it affects all version of IE6.