Fullscreen mode is broken. Use the View menu to switch to Floating Windows or run the software in a Windows XP virtual machine for full reliability.
Altium sometimes offers legacy licenses to existing enterprise customers. If your company has an active subscription to Altium Designer (the modern successor), you can contact support to request a legacy license key for Protel 99 SE to open old projects. Protel 99 Se Download
The most common reason for needing a Protel 99 SE Download is to access old data. You don't need the software; you need a converter. Fullscreen mode is broken
How to convert .DDB to .SchDoc / .PcbDoc for free: Protel 99 Se Download
Rarely, distributors who sold Protel 99 SE years ago might still have a private FTP archive. If you maintain a corporate account with a legacy Altium reseller, you can request access.
Unless you are forced to open a legacy .DDB file, there is almost no reason to download Protel 99 SE in 2024.
If you are searching for "Protel 99 SE Download" today, you are likely looking for a way to open old archived projects or attempting to learn PCB design on a budget.