I had a similar problem a couple years back when I still used Outlook Express and it drove me nuts. Eventually I deleted it through Add/Remove Windows components, restarted then re-installed it the same way, Add/Remove Windows components. You will probably be prompted to load your OS disk to re-install it. But the point was it worked fine after that, just made sure I configured it properly with the right pop and smtp configuration.
Never did really get down to finding out exactly why it did what it did, I was just looking for a quick fix and took a chance that would do it and it did.
I use Outlook Express 6.0. I can't delete email messeges or block them. My list of emails is getting too long. I tried Microsoft's help sites, but it seems nobody else has had this problem.