With very few exceptions, if you design a Windows program correctly it should continue to run on new versions of Windows ad-infinitum. But Microsoft also adds new features with new major versions and you may want to incorporate them. Alun Jones, a Microsoft Security MVP, author of the Tales From The Crypto blog, looks at this issue with respect to a program he writes and sells, the WFTPD FTP server. He's decided to "Vistafy" it. Interestingly, none of the changes he proposes (as far as I can tell) would break the applications on other current versions of Windows. He's mostly talking about changes, like moving away from CPL files for the Control Panel, that make a program work better in Vista, and which conform to best practices. It's all worth considering for your own apps.