Windows automatically shutting down at login screen

I recently purchased the parts for a new custom machine and put it together and installed Windows 7 x84. Initially there were no problems and the machine would happily boot into windows and allow me to use the system with no hiccups. However, after I began downloading and installing updates for Windows and the graphics card on subsequent reboots the machine would only boot to the login screen and then immediately change to the 'Windows is shutting down' screen. No error messages of any kind, it simply went straight to shutdown mode.

Also, if I rebooted using 'Last known good configuration' the machine would start up fine again. However, I was unable to boot to any mode other than this. I have tried restoring the system to a previous working condition to no avail, and none of the windows diagnostics tools can find any problems.

Here are the specs for the machine:

Intel i5-750 cpu
Gigabyte P55-UD3L Motherboard
2x2 gig G.Skill ram
Samsung 500 g hard drive
Sapphire Radeon HD 4850 graphics card
Seasonic SS-550HT power supply

Any help at all on this would be greatly appreciated, as I'm at my wits end trying to find a solution for this through searches.

edit: small update

I've now reinstalled Windows 7 5+ times with the same results every time. Every time I MUST reset all BIOS settings to factory optimized defaults and delete and repartition the hard drive or Windows WILL NOT install. If anything is left on the hard drive it fails to install, and if ANY changes are made to BIOS it fails to install. This includes things as basic as changing the boot order for devices (such as cd rom being first priority to HD being first). During these failures I get error messages as follows: 'Windows cannot be configured to run on your systems hardware." If I do all these things(resetting BIOS and reformatting all partitions) I can successfully install Windows and boot it up, but inevitably after the second or third restart of the machine it goes back to automatically shutting down at the login screen.

another small update:

As of right now I am on my sixth reinstall and now Windows will not install no matter what changes I make to the BIOS or HD.

As an aside I also cannot install XP Pro seemingly no matter what changes I try.

Might it be possible I have a faulty MB or HD, or is there some quick and easy fix I am missing?

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