Windows was simply not designed for this kind of fine grained security, and so what you want to do is largely impossible.

Your best bet would be to use ActiveDirectory if you want to continue using Windows. I can't say for sure if even it would permit you to do what you want, but if it can't, then you're completely out of luck on Windows.

Also, you really shouldn't use Outlook Express. It is the Internet Explorer of email clients, just like it's big brother Outlook. Very prone to mass mailing email worms, viruses, spam, etc. Out of the dozens of other email clients that aren't Outlook or Outlook Express, pretty much any one you pick would be considerably safer. Plenty of good ones are even free, like Mozilla Thunderbird, and I think Pegasus Mail is still around.