They don't do their yearly cleaning. The machine has to work a little to play video and it's game over.
Example cleaning at http://www.cnet.com.au/laptops/laptops/0,239035649,339274654,00.htm
A friend has a Dell laptop running Windows XP with 512MB of RAM and plenty of hard drive space. Whenever she tries to watch a YouTube video, it freezes. Any suggestions would be appreciated.