Actually - the worm is not spread by MS Messanger, in fact there is no security flaw in either Messager, IE, Outlook or Windows involved here. The end user must EXECUTE a file themselves for this to spread (at least under any reasonably up to date WinXP system). The involvement of Messanger is limited to sendign peopel on your buddy list a message that contains a URL for them to click. That URL will then attempt to trick them into running a file with the worm in it. In other words, MS Messanger is absolutely not a source of vulnerability in this attack.