A new report by Microsoft shows that the two most prevalent threats to Windows PCs in the first half of 2009 were malicious programs that have been aided mightily in their spread by a decision by Microsoft to allow the contents of removable media — such as USB thumb drives — to load automatically when inserted into Windows machines. In its latest “Security Intelligence Report,” Microsoft counted the number of threats detected by its anti-malware desktop products, and found that the Conficker worm, along with a Trojan horse program called Taterf which steals passwords and license keys for popular computer games, were detected on 5.21 million and 4.91 million Windows computers, respectively