Outlook Express
http://http://www.microsoft.com/ie/security/oelong.htm
For Netscape Mail users, a fix will be included in Communicator Version 4.06, which is due on August 7, according to officials in Mountain View, Calif.
In the interim...Netscape Mail users should be wary of messages with attachments from unknown users...Delete such messages and close the program directly with the "X" button rather than exiting the program through the File menu.