Computer shutting down during bootup

At what part of the boot up -- is it during the loading of windows ?? - can you get into the BIOS and does it stay on when you access the BIOS screens ? -- If so what RAM are you using and is the voltage and speed set correctly (Most newer DDR3 MOBOs will default to 1.5V for the memory and some RAM modules require higher voltages to remain stable in windows so check what voltage and speed of RAM you have and ensure that the settings are correct -- also what CPU do you have and does the MOBO bios that you have support that CPU and does it require a specific version of the BIOS to support it ?? (many AM2+ MOBOs need newer BIOS flashes to support newer CPUs) - If the MOBO does not support the CPU under that BIOS version it could cause similar problems.

But to troubleshoot would require having more info on the system and more specific details of the problem !!

Thanks for the reply. This computer has been working fine and just started doing this the other day. It is an older machine so Ill try and give all this information I can on it. It reboots after the window screen comes up if it makes it that far. I can get into BIOS and it stays on while in there. It is running on DDR2 ram i do believe and the memory voltage is set to auto in the BIOS. If you highlight it a red alert flashes and says - not recommended system may be unstable, not sure if that is standard because I haven't checked that in awhile. The NF4 Voltage is set to 1.5v. The CPU is an amd 64 3500+. Nothing has been changed as in a good while on the machine for quite some time now so I am puzzled.

If you need anymore info Ill do my best to get it.

?????

Just to clarify; the system passes the POST, (Power-On Self Test) starts to load Windows and then, it shuts down or restarts? Also, can you list as much as possible, the specifications of the computer?  

Yes it shuts down when the windows screen comes up. I am thinking it is a hardware problem but not positive. it was custom built at least 5 years ago. There has been some upgrades but there hasn't been any made to it in at least 2 years. The computer has been working fine with no problems. The green light that flashes after it shuts down is the green light that comes on when the computer is turned on. I am beginning to believe it is hardware related but don't know where
to start or what could cause the problem.

It has an amd 3500+ processor with an msi motherboard. Not exactly sure which model but I can try to figure it out. Has ddr2 ram and running windows xp.

It is also running on the K8N Neo4 series motherboard. I believe it is the MS-7125

Starting to look like harddrive failure, do you have another hd lying around just to see if system hangs.