Security Fix has learned that at least one of the security holes in the popular Adobe Reader application that was quietly patched by Adobe this week is actively being exploited to break into Microsoft Windows computers. On Wednesday, we alerted readers that Adobe had pushed out a patch to plug unspecified security holes in its ubiquitous and free Acrobat Reader program. According to information released Friday by iDefense, a unit of Verisign, Web site administrators on an online Italian forum first spotted hackers taking advantage of the flaw on Jan. 20, 2008, when tainted banner ads were identified that served specially crafted Acrobat PDF files designed to exploit the hole and install malicious software. iDefense says that on Friday it saw the same banner ad tactic being used in the wild to install a Trojan horse program. That Trojan, dubbed “Zonebac,” disables various anti-virus products and modifies the victim’s search