Well the new motherboard arrived this week, so it got installed and improved the situation. It failed to solve it though. The new motherboard enabled the system to post info on each reboot, even though it proved not to be the solution it enabled testing of the other possible causes much much easier.
Testing of the hard drives initially showed them as ok but I wasn't planning to trust them for too long. The plan was to free up space on the larger less often used drive and copy the C: partition from the failing drive and run it off there. The problems began in the freeing up of the space. Partition Magic managed to corrupt the file system meaning data that i had not backed up seemed to be lost. A little recovery program later and I had access to it. What followed was a mad backup session with data being shifted to other computers for safekeeping. A furthur test on this drive yeilded a component failure, making that two failed drives, both out of warrenty.
Today I went and purchased two new drives at great expense to solve the problem. I managed to copy the C: partition across and on we went with booting it. Oh dear, same reboot issue. Several checks of the file system yeilded no clues and things were looking grim. A clean install might be the solution, it wasn't, it didn't even make it through the installer before it rebooted.
A last ditch effort i looked at the discussion forums for my Asus motherboard. Similar problems kept cropping up, the cause seeming to be excessive harddrive activity caused by games and large data access. Then it came to me, drives draw more power when they are worked more heavily. I connected the hard drive to another power supply and resumed the installation, it passed the point it had rebooted at twice, a successful install. Swap the drive with the old one, that too booted fine at last.
The problem is the powersupply, thats why the system would not reboot when I first powered it down. Thats why the hard drives were making odd noises (not enough power). Thats what caused damage to the motherboard and the drives themselves. A new power supply is on order to arrive early next week. That should solve the hardware problems, just leaving a few software tweaks to get it happy with the different hardware.
I'll post another update once the new power supply is in.