First, you need to have antivirus software installed on your system and you
should be sure to have the latest updates.
Second, when an e-mail comes into Outlook Express, you should see an icon
representing the unopened attachment, depending upon your resolution you may
see it as something similar to a paperclip. Right click that, select "Save
As, " save it to some temporary location outside your e-mail, a temp folder
will do, then right click the file in that location, most antivirus software
also provide an option to that menu to scan the selected file, do so. If
your antivirus doesn't create that option, open your antivirus software, it
should give you an option to scan selected files, select that file and click
scan.
While many antivirus applications now offer the option to scan e-mail, they
may miss the attachment so it's always best to be cautious and scan that
attachment. With potential viruses, it's better to be safe than sorry.