I have downloaded a .mp4 video from ITV Anglia's website but Windows XP won't play it. John H Fagg
Microsoft has been shockingly derelict in providing MP4 support and its website directs you to a third-party codec (http://support.microsoft.com/kb/316992), though mp4 files can also be repackaged as .asf files.
Instead, I recommend downloading the K-Lite Codec Pack (http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm), which includes the open source Media Player Classic (don't download this separately). This will play MP4 videos and just about anything else. If you also get the QuickTime Alternative and Real Alternative add-ons, you don't need any other bloatware players. An alternative is the cross-platform VLC (http://www.videolan.org/vlc/), which also has Mac OS X and Linux versions.