This is a wonderful new "feature" Microsoft decided that they ought to put onto your system.
Assuming you are using a new version of Outlook Express, you should be able to go to Tools-->Options, select Security, and uncheck Do not allow attachments to be opened that might be infected. You can look at
http://support.microsoft.com/default...;en-us;Q329570 for more information.
At least they put in a way to turn it off...before you had to edit the registry to do that...
Note that if this doesn't work, I have been able to access my attachments by choosing to Forward the message...then it lets you save and open the attachments (at least that works in regular Outlook).
hope this helps...
~psi42