MS06-049 is not categorized as virus, worm, Trojan or backdoor. It is an important vulnerability in the Windows Kernel on Windows 2000 computers, which allows local privilege escalation in the vulnerable computer.
If exploited successfully, MS06-049 allows to gain unauthorized privileges on a computer or network. An example of privilege elevation would be an unprivileged user who could manage to be added to the Administrator's group. In such case, 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 running a specially crafted program in the vulnerable computer. In order to do so, a hacker must be able to log on locally to the system.
If you have a Windows 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.
Additionally, bear in mind that this security bulletin replaces the previous bulletin, called MS05-055.