Security researchers have published a proof of concept program modeled after an actual attack that shows how hackers can user Microsoft Window's Background Intelligent Transfer Service (BITS) to download and Update Malware.
BITS is used Windows for automated security updates. BITS constantly searchers for updates and is designed to resume downloading unfinished files even after a machine is restarted or a user logs off.
Hackers who are able to sneak a trojan onto a users machine via an email or website can use BITS to download additional malware and update it. Because BITS is a legitimate, trusted application, inbound and outbound malware using BITS is unlikely to be stopped by the system's firewall.
Researchers note that the attack will likely fail on any machine where the user is NOT logged in as an administrater. This brings up a point that we constantly strive to make--one of the best defenses against viruses and malware is to only give administrator privelages to users who absolutely need it and to only be logged in as an administrator when it is neccessary. If every business and home user followed this advice, the efficacy of virus and malware campaigns would be slashed dramatically.