Certainly, most application software [u]can[/u] be written in such a way that it runs on all versions of Windows. Mozilla Firefox is an excellent example of this (Windows 9X, NT, 2000, XP, Mac OS, Linux, Unix). In order to achieve this, Mozilla developed the Mozilla Application Framework. But...
- Microsoft calls it "[u]the[/u] Win32 API" and "[u]the[/u] .NET Application Framework", but there are MANY versions of these and many incompatibilities.
- There are numerous programming (API) differences between Windows 9X and NT4/2000/XP/Vista. So, many developers/vendors completely write off Windows 9X because of that. The theory is that anybody still running Windows 9X is not buying new software. This is largely correct.
- There are some programming differences between Windows 2000 and XP. Since, Windows 2000 is "ancient" in computer terms and no longer even supported by Microsoft, many developers/vendors write it off. (Windows 2000 was/is a perfectly fine system as far as I am concerned and I upgraded to XP reluctantly years ago).
- You're absolutely correct about Microsoft's applications. Once the new operating system is out, all new MS applications only run on the new OS. On the other hand, the new MS applications often take advantage of new features in the new operating system. As you know, Microsoft's revenue model depends on upgrades to new versions, not maintenance of old versions.
- Vista will be no different in this regard. However, previously Microsoft said there would be a significant Windows XP Service Pack that enables new Vista applications using "Avalon" (presentation) and "Indigo" (communication) APIs to run on Windows XP. But, it appears this (SP3) has been delayed until the first half of 2008! (1)
- This is just one reason why I avoid MS software development tools. The entire MS "machine" is oriented around [u]forcing[/u] everyone to upgrade to the newest releases on Microsoft's terms.
So, you're absolutely right! GTH MS!! X-(
On the other hand, upgrade to the latest OS or quickly fall behind. :-(
(1) Windows Service Pack Road Map
http://www.microsoft.com/windows/lifecycle/servicepacks.mspx