Google Microsoft Search is one of the topic specific searches that Google offers. I'm not sure when Google added these topic specific searches because I just discovered them (yes there are more) when using the Advanced Preferences from Google.
The first of them is Google Microsoft Search.
As the name suggests, what it allows you to do is to search Microsoft-related sites using Google. I think it is a normal google search but with some filters added (like +microsoft -linux -apple), given the fact that if you do a search using it for -microsoft you get 0 results, while a similar search on Google returns about 9 billion results.
If you're asking why does Google do that, well here there is place for interpretation. Of course that this helps you find results only in websites that are related to Microsoft and its product/services. But there is also the optimization issue, search for Microsoft on Google and see who's on the second page (and going up). Of course this is a supposition ;).
Another topic related search is the Google Apple Macintosh search. Using this you can easily search for Apple/Macintosh-related sites using Google.
Search web pages about the BSD operating system using the Google BSD Unix search:
Since we're at this chapter, if I'm not mistaken BSD is about to change the logo with the new one, that looks like this:
Back on the track here, another topic related search is the Google Linux search, that allows you to "search all penguin-friendly pages"
Last (but not least) is the Google U.S. Government search, that allows you to search all .gov and .mil sites
You can find all these topic related searches in one place on the Special Google Searches page.