Because MS change their instructional code set with each Windows version most third party software, and some MS software, isn't compatible between versions. So you need to get new copies and shell out more bucks.

In XP most of my older Windows software and games won't run, yet they work perfectly in Linux in either WINE or Cedega.

I have friends at Uni who play NWN and a few other on-line games, the games are written for Windows based systems; yet they all say they see a better game performance on Linux using Cedega than on XP.

I know many small businesses that are converting to Linux as it's cheaper for them to buy Crossover to run their few critical Windows applications in than buying the latest versions because their existing versions aren't compatible with Vista.

So it all comes down to how much you want to pay and how much trouble you want to go through is 5 or 6 years when the next Windows version comes out.