Most people are familiar with the notion that a computer virus can be passed from PC to PC, but many folks would probably be surprised to learn that a sick PC can often pass its infection on to Web sites, too. Some of the most pervasive malicious software circulating today (e.g., Virut) includes spreading capabilities that hark back to the file-infecting methods of the earliest viruses, which spread by making copies of themselves, or by inserting their code into other files on the host system. Malware often modifies existing files on the victim’s PC to maximize the chances that infected files will be shared with and downloaded onto new host systems. One of the most effective ways of doing that is for malware to inject copies of itself into all of the HTML files found on a victim’s computer. The end result could be this: If the victim is also