Google employees have until now been able to choose their OS of choice, but apparently due to security concerns in the wake of that whole hacking mess, the Google higher-ups have deemed Windows too risky to support. Google declined to comment.
Google competes with Microsoft in several areas, not least of which is Google's bread and butter, search--Microsoft's Bing is gaining marketshare (though, to be fair, it's nowhere near Google's). The two tech giants also compete in the cloud, with Google Docs taking on Microsoft's online version of Office, as well as in the mobile world (Android vs. Windows Mobile). Soon, they'll have even more competition; Google TV plays in the same sandbox as Windows Media Center, and Chrome OS may compete for space on netbooks with the lightweight Windows 7.
On the other hand, Apple is at least as fierce a competitor as Microsoft, and significantly more so in the mobile world, yet Apple's full-sized OS seems to be the norm at Google HQ.
Google may not have intended this move to look like a dig at Microsoft. But when a company as high-profile as Google finds Windows too easily attacked to be used by its employees, it's got to sting.