No 100% safe source exists today. Even CNET’s old download now redirects to newer software. Use at your own discretion.
Downloading YouTube videos violates YouTube’s Terms of Service (ToS) unless the video is your own or has a download button. However, fair use arguments include: xilisoft download youtube video 3.2.1 download free
If you absolutely need that specific old version for archival or testing purposes, run it in a sandboxed environment (e.g., Sandboxie or a Windows XP virtual machine)—and never enter any personal credentials. No 100% safe source exists today