A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Disinfection & Removal
Trojan-Downloader:W32/Scob was found on a number of web sites on late June 24th, 2004, appending to existing files such as jpeg and gif files. According to reports, the script was not appended by modifying the actual files on the server but by using the footer feature from Microsoft's Internet Information Server.
When executed, the trojan attempts to use an invisible frame to connect to a page at a remote web site. At the time of writing, the page in the web site is not available. While the page is not currently available, there has been reports that this downloader has been used to install variants of Backdoor:W32/Padodor.
The trojan also sets a cookie on the system, causing that it will attempt to connect the remote site no often than once every week.
Further information about this case is also available from Microsoft:
In addition Microsoft has released a new KB (871277) on the Download.Ject Detection and Recovery Advisory: