Automatically rebooting every hour

Hi Experts,
Here is a strange one that has me scratching my head.

The problem has occurred on 2 identical PC clone systems (One with Windows Vista Business and the other Windows XP pro)

The hardware specs are - intel DG33FB Motherboard with Intel Core2 Duo E6750 processor, 2GB Kingston DDR2 667Mhz RAM, Samsung 320GB SATA HDD

Here is the scenario - The PC is working fine when suddenly; one of the PC fans inside the case audibly increases fan speed and a 4-5 seconds later the system resets. Once the system is back online the same thing is likely to occur EXACTLY one hour later. This may repeat for up to 10 more hours and then the system may be fine for a few weeks.

Notes - When the system reboots, nothing is even slightly hot. I have run the intel system monitoring software and all temperatures are in the green before the system resets. I have taken the case off the PC to allow greater airflow but this made no difference. There is nothing in the event log that indicates why the system has reset, only that classic line "The previous shutdown was unexpected".
This is in a corporate environment, the only application software loaded on the system is
Office 2000 SP2
Ouitlook 2003
Mcafee virusscan 8.5
Winzip 9.0
Abobe acrobat reader 7.0

Thanks for any direction.