There are plenty of ways to do a search on Google, but if you want a specific result that is targeted to your OS, then you might want to give Google specific search a try.
Google specific search allows you to search within a specific topics, especially topics pertaining to a particular OS. For example, if you want to perform a Mac-specific search, you could just head on to Google Mac Search and type in your query. Google will only display result only related to Mac.
Some other useful OS related search sites included
Linux: http://www.google.com/linux
BSD: http://www.google.com/bsd
and not forgetting Windows: http://www.google.com/microsoft.html
For other specifc search, you can find the listing at http://www.google.com/options/specialsearches.html
