Alan
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Is it a system reboot? Does this happen only in Heroes, or in other games as well? It could be overheating, or a weak power supply, or maybe your system just isn't compatible with Heroes. Send Nival a bug report to technical support.
P.S. I think there was a thread here about problems with the 6600 GeForce.
Тьма - это то, что позволяет смертным не забывать, как ярок бывает свет.