Join the Windows CE dev forum at HPCFactor.net or the #ce-os channel on freenode (now Libera.Chat). The community maintains a private archive of all Platform Builder rollups and BSPs. Ask nicely, prove you own a license, and they will point you to a download that works.
For those specifically needing the Platform Builder environment for legacy maintenance: Windows CE 5.0: Standard Software Development Kit (SDK) microsoft windows ce platform builder 50 download work
If you are looking to get this legacy environment operational today, here is a comprehensive guide on the download status, installation requirements, and how to make it work on modern systems. Join the Windows CE dev forum at HPCFactor