XMLSpy scans a specific key in the Registry of the user each time the application starts:
HKEY_CURRENT_USER\Software\Altova\XML Spy\PlugIns
All values of this key are treated as plug-ins and should conform to a specific format. The name of the entry is the ProgID and the data is equal to the CLSID of the component. The type of the value must be REG_SZ.
For each PlugIn the user adds to XMLSpy using the "Customize" dialog, a new value is created for this key. You can also add these values from outside of XMLSpy using any other method of writing to the registry. No additional information is necessary to register a plug-in for XMLSpy. The new plug-in will be activated the next time XMLSpy is launched.