Though not necessarily a virus, a Trojan horse is a type of file that seems to do one thing but actually serves as a way for unauthorized access to gain entry into another computer. Once access is gained, the unwanted intruder can access files, watch the screen and even control the computer. A Trojan downloader accesses a file from a remote computer and then installs it on your computer.
Usually a Trojan will appear as a normal program that the unsuspecting user downloads. Once it is run on the system, the designer then has access to that machine.
How It Works
Once running, the Trojan then uploads hidden programs, such as scripts and commands, onto the computer.
All Trojan downloaders are different, so there is no one method to get rid of them. Clearing out temporary Internet files and running a virus scanner is good way to remove most, but not all, of them.
According to Symantec, the threat of a Trojan downloader is low and the containment and removal is easy (see link in Resources).
Keeping your computer behind a firewall. Change passwords often. Do not download programs from unknown sources.