This is a trojan virus affecting Windows XP and Windows Vista.
If the trojan infects a computer it will copy itself to files as well as creating registry entries so that it can begin during startup.
The trojan will then attempt to download files to the infected computer from remote locations. Generally it will download further malware onto the computer, or programs such as backdoors, keyloggers etc, for the purpose of obtaining private information and other personal data.
Trojans can operate in the background, without the user being aware. Always scan your PC on a regular basis.
