Security-software makers may not yet see Google ( GOOG) as a rival. But the search giant's $625 million decision to acquire Postini , a privately held company that offers email filtering and compliance services, is firing straight onto security's main turf. Google's latest acquisition is being seen as a move by the company to bolster confidence of businesses in Google Apps, which brings Gmail, calendar, docs and spreadsheets, and a Web page creator into a single suite. Postini is not the first security company that Google has bought. Two months earlier, it acquired GreenBorder, whose software helps create secure Web browsing. And Google's toolbar already warns users about Internet malware and has extensions to detect phishing, fake Web sites that try to steal user information. Google is also partnering with Stopbadware.org, an organization that indexes Web sites as those containing potential infections. Google's growing security ambitions is, primarily, a frontal attack on Microsoft's ( MSFT) stranglehold on the personal computer. At the same time, Google is trying to better control the user's Internet experience, so it can predict more accurately what a user needs in an effort to better customize advertising. Potentially in the way of this war of titans could be traditional security companies like Websense ( WBSN) and McAfee ( MFE), which have spent millions on acquisitions to build products similar to what Google is focusing on now.