Microsoft released a new version of ACT, 5.6, last week. ACT will provide you with the tools to help “fool” incompatible applications into working on Windows Vista or Windows 7.
When doing an upgrade, you’ll identify those applications that won’t work on your new OS. You would try to either fix the application or get a new version that won’t work. Maybe you’ll try to find an alternative application. If that’s not possible (no budget, vendor is gone, developer is gone, etc) then you would try to get the application working. ACT works by providing shims that sit between the OS and the application. For example, an application may not be UAC aware and you make it aware via a shim. It might explicitly check for Windows XP so you use a shim to lie to the application about the version of the OS. It is pretty cool stuff.