The Internet Security Threat Report says in the first half of 2004 there were at least 1,237 newly discovered software vulnerabilities. That translates into an average of 48 new vulnerabilities a week. The average size of a bot network was about 2,000 PCs known as zombies. There was one network of more than 400,000 such machines. The survey established that 50% of the attacks came from captured computers with Internet addresses controlled by Fortune 500 companies. The survey also documented more than 4,496 new Windows viruses and worms during the most recent period, which is 4.5 times the number from the first half of 2003. In January 2001 it identified only 308 malicious programs.