Can you get into the BIOS?
Does it beep once (go through POST) then restart again?
When you start up, does it show all the hardware info (MB, BIOS, RAM, IDE, SATA) then it restarts?
Try booting with only one stick of RAM in a different slot (rather than DIMM1) and see if you are able to load Windows.
If you can then a RAM slot could be bad (bad MObo)
If you still can't boot in Windows, unplug the HD (put the RAM back in) and see if the system still goes into a reboot loop or not.
If it doesn't, then your HD could be dead.
Do you had another video card? You can try plugging in a video card you know that works. Or if you have onboard try that and disconnect the video card you have plugged in.
That's what I could think of for you to try. Sorry if i listed ideas that you have tried already
