MS04-013 is not categorized as virus, worm, Trojan or backdoor. It is a critical vulnerability in Windows 2003/XP/2000/NT/Me/98 computers, known as MHTML URL Processing Vulnerability, which allows to remotely execute arbitrary code in the vulnerable computer.
This vulnerability affects the following products:
If exploited successfully, MS04-013 allows hackers to gain remote control of the affected computer with the same privileges as the logged on user. If this user had administrator rights, the hacker could take complete control of the system: create, modify or delete files, install programs, create new user accounts, etc.
This vulnerability is exploited by creating a malicious web page and enticing users into accessing it. An e-mail message in HTML format could also be used.
If you have a Windows 2003/XP/2000/NT/Me/98 computer, it is recommendable to download and apply the security patch for this vulnerability available in the Microsoft Security bulletin. Although the information related to this vulnerability is in the MS04-013 bulletin, you should rather apply the patch MS04-018, which is the latest cumulative update for Outlook Express.