In such case you can download a tool like VLC mediaplayer to check and playback your downloaded videos.
Microsoft has discontinued the support for Windows XP, which means that innovations on web pages are no longer supported. Specifically, from sites using the https protocol with new certificates, e.g. Youtube, you cannot download videos under Windows XP.
- Start Internet Explorer (even if you otherwise use a different browser)
- Enter the menu Tools->Internet Options
- Click Connections
- Click on Lan settings
- Disable the checkbox under proxy server
- Start the Task Manager (right click on an empty area of the taskbar)
- Stop the process dllhost.exe
- Delete the last downloaded video