If you're still using Visual Studio 2003, you might want to consider alternative development environments:
: Some users report that to avoid "Source file not found" errors during installation from ISO images, it is best to extract all discs into a single directory before running Compatibility Mode
Keep in mind that Visual Studio 2003 is an older version of the software. You might consider upgrading to a newer version of Visual Studio, which often includes improved features, security, and support.
Searching for a "Visual Studio 2003 product key" is a symptom of a deeper problem: technical debt. The software development world has moved on. The keys that once unlocked the future of .NET are now relics.
If you have an active Visual Studio Subscription (formerly MSDN), you can sign in to the portal, navigate to the Product Keys tab, and search for "Visual Studio .NET 2003" to claim or view your key.
Finding a product key for software that is over 20 years old can be a challenge. Here is how users typically navigate this for archival purposes: