Visual Studio 2002 is built on and runs on Framework 1.0. Visual Studio 2003 is built and runs on Framework 1.1. You can have each installed at the same time and each will run on the appropriate framework, but neither will run on the other framework. This is by design.
However, applications built on one framework can run on the other framework if there are no backward or forward incompatability issues.
Here is another thread that discusses this topic.
Peter
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Work smarter, not harder.