Not withstanding that comments above and the need to test all business functionality.

If you are looking to identify potential areas of risk then the following might help:

I would expect any are of your application the interacts with the operating environment to be at higher risk after the migration. These areas are typically as follows:

1. External file writes/reads
   1. Permissions & security changes that prevent these operations.
2. Call to external DLLs
   1. Permissions & security changes that prevent these operations.
   2. External DLL function changes that affect the system logic.
3. OLE, COM, Run Command or other access to third party software.
   1. Permission & security changes that prevent these operations.
   2. Other environmental changes that effect the running of these programs.
4. Other Windows interoperations and drivers software
   1. Graphic drivers - screen refresh issues and display
   2. Calculation results affected by processor changes
   3. odd / unexplained results.

The last one is what really requires the full regression test but even still they do not manifest themselves until you are running in a production environment. Best endeavours and all that, you cannot test for everything :)

Targeted intelligent testing is the most cost effective way to ensure minimal risk.

Hope this helps.

Regards

David

Every company I have worked for would perform a full regression test on any internally developed application which was placed on a new version of windows.

Regression testing is the process of testing changes to computer programs to make sure that the older programming still works with the new changes.Regression testing is a normal part of the program development process and, in larger companies, is done by code testing specialists.