MS05-049 is not categorized as virus, worm, Trojan or backdoor. It is a group of important vulnerabilities on Windows 2003/XP/2000 computers, which allows arbitrary code to be remotely executed in the vulnerable computer.
If exploited successfully, MS05-049 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.
These vulnerabilities are exploited in different ways, though, in general, they are related to the way in which Windows processes determined files. For further information, refer to bulletin MS05-049 of Microsoft official website.
If you have a Windows 2003/XP/2000 computer, it is recommendable to download and apply the security patch for this vulnerability. Click here to access the web page for downloading the patch.