If you own a physical disc, you are golden. However, the installer on that disc (circa 2010) lacks patches for modern OS. You’ll still need to follow the tweaks in Part 4.
This specific version is often missing on new PCs and is required for Mathcad 15 to run.
Visit the official PTC Community Mathcad forum or search for “Mathcad 15 Installation Guide PDF” for a printable reference. Good luck with your engineering calculations