RedmongMag has a column in which they explain that the biggest Windows Vista competitor isn't Mac OS X or one of the dozens of Linux distributions. The real rival is the old Windows XP:
Microsoft is currently facing some of the same problems with Vista it has been experiencing with Office for a couple of years now. The biggest competitor to Vista isn't Mac OS X or Linux -- it's Windows XP. Consequently, the Windows team increasingly finds itself in the same straits as the Office folks -- namely, it needs to convince users who don't really need a brand new release of Windows that they do. Let's see what Sinofsky & Co. come up with, beyond making new, desirable features available only to customers who sign multi-year volume- licensing contracts.