Problem running .NET application

Question

This is making me go nuts!  I worked for a long long time on this project, but on computer with .NET 2.0 it refuses to run, and now it doesn't even run if 2.0 is uninstalled and 1.1 is installed.

The only computer I can get it to work on is my old computer that it was developed on!

On other computer it has the following error Class not registered!, Object reference not set to an instance of an object!

The person I am developing the project for could run it until he uninstalled his .NET runtime and installed the new one!  After that it refuses to run!

This really pisses me off as I heard .NET was backwards compatible, I guess not.    If someone could help me I'd be more than happy!    It's close to a crisis here :(

Thank You,
Jonatan