I had a client that'd paid for a custom Microsoft Access application and didn't want to dump it. The original (non-professional) Access developer wrote it to run under Access 97 and didn't package the application with a run-time version. So everybody had to be running full Access 97 on their PC. The application wouldn't run under MS Access 2003, and the developer didn't know how to convert it to run.
By the time I got into the picture, the required Access 97 was four-generations old. It had to be installed on every new PC. Every time a PC arrived, I had to REMOVE Office 2003, install Acess 97, delete some fonts (believe it or not, this is required when you need to install Access 97 onto a recent PC), and then re-install Office 2003. It added an hour to the installation of every new PC.
The "lesson" that I came away with is that it's probably best to fix custom applications as incompatibilities show up. I don't think that Vista is going away.