I don't think all that many average users will move to Apple Macs, or Linux in the near future. Currently Windows operating systems are the most popular by a long way, and thus, people know them, and know how to work them a lot better; I doubt you will
see many people deciding that using Mac OS X, or a distribution of Linux will be eqaul to using Windows XP for example.
The majority of people who buy computers from Dell, HP, etc. will continue to get Windows XP, Windows Vista, etc. pre-installed, and they are unlikely to reinstall the operating system. People who play games will use Windows operating systems, and people who
need to run specific applications will probably run a Windows operating system due to compatibility.
I don't think Microsoft has anything to worry about, advertising Windows Vista doesn't have to be done too much, the computer manufacturers do that when they try to sell people the most expensive system they can (make most money), which in a few months time
will run Windows Vista.