Dell 670 restarting every Windows boot

I have just found myself with an old dell 670 machine. With the future plans to upgrade the machine with a nice amount of RAM, it is housing an Nvidia quadro 3800, and would like to get it up and running. With the major issue of the boot of windows forcing a restart upon every start up I would like to solve this issue. Currently I have available 4 x 512 RAM sticks which I have swap around in different configurations, just to ensure that it is not the RAM causing an issue. This seems to be ok as the bios shows that the RAM is configure correctly. The system makes it to the safe mode screen, which I suspect means that the the HDD contains system files that can be read by the machine. Regardless of what option is selected at this safe mode page the result is always the same, the systems restarts and starts the cycle again. So far I have assumed that it could well be the system files on the HDD that are causing the issue. So I have tried to re install window XP, the installation gets so far, and then shows a message that there is no HDD available? Confusing. With this being the case, I suspect an issue with the motherboard, but then again why would the safe mode allow me to attempt to run WIndows if the HDD is not being read properly by the motherboard. I would love some help with this. Mainly cause its irritating not knowing the reason behind it, I can always salvage components. Regards,

When you say you swapped the ram around, did you try removing all but 1 512MB module and see what happens. If that fails, try it with a different one.

Check the hard drive with the manufacturer's diagnostic tools.
Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp.
Bootable Hard Drive Diagnostics

Being able to boot to Safe Mode doesn't mean the drive is good.

Does it have and onboard display connector? If so, try removing the Nvidia Quadro 3800 and use the onboard, just to see if that works. If yes then the Nvidia may be bad.