The story goes on to quote several employees claiming that clearance from "senior levels" is required to stay on Windows and that Microsoft's operating system is distrusted by definition at the company. Although Google has declined to comment on the matter, something feels greatly exaggerated about this so-called 'purge'. For starters, the largest share of Google visitors is Windows-based, so no matter how web-centric and platform independent the company aims to be, it still makes sense to prioritize development and thoroughly test their products on Windows.
Microsoft's operating system and its Internet Explorer web browser have a reputation for being vulnerable to attacks. It should be noted, however, that the system compromised at Google back in January was running Internet Explorer 6 on Windows XP and logged in as Administrator. In other words, at fault was a 9 year old browser on a platform that is two versions out of date. It doesn't seem like a serious effort at security was really made.
Microsoft has made some significant security improvements in the newest versions of Windows. On the other hand, Linux and Macs, while not necessarily more secure, in most cases don't have enough market share to justify coding malware for them. The Windows ban -- if real -- probably isn't company-wide and may be at least in part a way to promote the use of Google's upcoming Chrome OS.