If it's playing a video from a web page, try the old FLASH DISABLE HARDWARE ACCELERATION (see google.)
But given only the clues so far, I hesitate to guess.
Computer is a generic PC desktop ACPIx86 Intel core2 cpu, firstname.lastname@example.org ghz. 2gb ram, 32 bit Vista sp2. Windows experience 4.7. It is 5 years old.
Have had three technicians run diagnostics tests on my computer and reinstall graphics drivers, reinstall IE, reinstall Active X. None has stopped the freezing. It will either go black and reboot, or I will reboot it. I ran a stress test on the graphics card, Invidia Ge Force 8500T and it did not fail. The hard drive is new. The freezes occur when I have only one internet site open or if I have four or more. Does anyone know of something else that may cause this?