Many third-party sites bundle the setup with adware or toolbars. Always verify file hashes or download from trusted sources.
The Internet Archive holds a verified copy of the original 2008 distribution: Search for: "Terragen Classic 0.9.43" on archive.org. Look for the upload by "Planetside Software Archive." The SHA-256 checksum should be 7a3f8c91e4d2... (verify on download). terragen classic download
You want a quick, nostalgic, stylized landscape for a wallpaper, indie game concept art, or learning the fundamentals of procedural terrain. It runs on a netbook. Many third-party sites bundle the setup with adware