As said, it could be a number of viri, however it sounds like Trojan/Nettroj.
Troj/Nettroj-A is a configurable and extensible backdoor Trojan. Infected hosts form a decentralised network that can be controlled by a malicious user.
When first executed the Trojan modifies several registry entries and INI files to become resident on the system.
The system files Autoexec.bat, win.ini, system.ini, wininit.ini and winstart.bat are modified to start the Trojan.
You will find recovery instructions here at Sophos:
http://www.sophos.com/virusinfo/analyses/trojnettroja.html