MS03-026 is not categorized as virus, worm, Trojan or backdoor. It is a critical vulnerability in the RPC interface on Windows 2003/XP/2000/NT computers, which allows arbitrary code to be remotely executed in the vulnerable computer.
RPC is a protocol used to request a service form a program located on another computer in a network.
If exploited successfully, MS03-026 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 usually exploited by sending a specially crafted RPC message to a vulnerable system over an affected TCP/UDP port.
If you have a Windows 2003/XP/2000/NT computer, it is recommended to download and apply the security patch for this vulnerability. In order to do so, Microsoft recommends to download the patch belonging to the MS03-039 bulletin, which corrects a mistake in the patch of MS03-026 bulletin.