As wombat suggested....there is just not enough memory to run Vista.

Vista requires a lot more memory because of the GUI (graphic users interface) than what XP required...about 2 times more.

Another thing could be that Microsoft broke the rules in designing Vista as it was/is not backward compatable as per computer protocol. In as such the hardware in your old computer may not work properly with Vista installed....even if the correct drivers are found and installed. Sometimes the firmware itself needs to be upgraded as well as the drivers. This would also cause the memory dumps to happen!