When it is a DVD. My mother ordered an update to her anti-virus software, and they sent it to here on physical media. The package said "here's your CD", and the disc was even labelled with her name. However, it was a recordable DVD. Her PC has a CD reader, not a DVD reader, so nothing happened when she inserted it: no error, just no action. Yet another Windows failure. FWIW, I downloaded the update software in seven minutes and got her updated in another ten.