Who would have expected that the biggest competition for Vista would be Windows itself. It is obvious, both anecdotally and statistically, that Windows XP remains absurdly sticky in the market and consumers see little reason to upgrade to Vista, and enterprises will force Microsoft to continue to support XP for a very long time. There’s a pricing lesson in here and a case study about the changing dynamics of the technology marketplace with regard to upgrade cycles for mature products.

What Microsoft is competing with is the absorbed-cost version of Windows XP that customers already have. But XP wasn’t actually free. Customers were bonked on the head to move from W2K (and I would say it was a good upgrade) and now are being strong-armed to upgrade to Vista, which has minimal upside. [From Vista's biggest problem is Windows XP | Negative Approach - CNET Blogs]