LyX is the closest modern replacement – same “write math as you see it” approach as SWP.

SWP 5.5 relies on MiKTeX (a LaTeX distribution). The installer will ask:

If you are looking to download Scientific Workplace 5.5 for Windows XP, you will face significant hurdles. Unlike open-source projects, Scientific Workplace was proprietary commercial software. It was expensive, often costing hundreds of dollars for a single license.