When testing application compatibility when moving to Windows 7 you can use ACT (Application Compatibility Toolkit) and the tools in there to test and fix applications. Another tool that you can use to learn about dependencies when compatibility testing your applications is a tool called Dependency Walker.
With this tool you basically open a system file, for instance a DLL or an OCX file, and it will list its dependencies to other files on the system. This can be good for finding what is causing registration of for instance DLL or OCX files to fail on Windows 7 while it works fine on Windows XP. There can be runtimes missing.