Remember that Outlook Express is a "freebie" that does have many small bugs. Cosmetic bugs are often not fixed since this email client is just a freebie. I know this may be hard to understand why they offer a buggy tool, but it's the nature of the industry. People new to the PC world expect better.
I know that sounds terrible, but I read your post and the glitch sounds to be cosmetic as you still have emails and it still works.
A lot about Outlook Express, fixes and backup is over at http://insideoe.tomsterdam.com/
I am running outlook express 6 & suddenly all my archived emails have developed a 'message forwarded' icon before them in bold type. I haven't forwarded any of these messages, so it shouldn't be there.
I have completed a virus scan (norton) without viruses being picked up.