Computer unexpectly shuts down

My computer just unexpectedly shut down just now.

There was no BSOD and it restarted itself after hanging at a black screen.

I don't see anything in my event viewer that would cause a problem.

I already set it to not restart again if it unexpectedly crashes again so I can write down whatever happened, if there is anything to write down.

Here's an error related to my anti virus at the time of my "restart", but it might not be relevant:

"The Eset Nod32 Boot service failed to start due to the following error:
The service did not respond to the start or control request in a timely fashion.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp."

also in security during time of restart.

"Logon attempt by: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
Logon account: MYNAMEHERE
Source Workstation: MYNAME
Error Code: 0xC000006A"

"Logon Failure:
Reason: Unknown user name or bad password
User Name: MYNAMEHERE
Domain: ETC
Logon Type: 2
Logon Process: Advapi
Authentication Package: Negotiate
Workstation Name: MYNAME"

Both are in my event viewer.

It could be something wrong, heat wise, but I don't see anything wrong when using speedfan. Most of the temperatures are around ~35-40C. I haven't over clocked yet and don't plan to soon.

I JUST built this computer recently. I don't think I did anything wrong hardware wise though.

Here's my system info.

System Information
Operating System: Windows XP Home Edition (5.1, Build 2600) Service Pack 3 (2600.xpsp.080413-2111)
Processor: Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz (4 CPUs)
DirectX Version: DirectX 9.0c (4.09.0000.0904)
Physical RAM: 3326MB
Virtual Memory: 2047MB

Video Information
Video Card: ATI Technologies Inc. ATI Radeon HD 4800 Series
Video Memory: 1024.0 MB
Chip Type: ATI Radeon Graphics Processor (0x9440)
Driver Filename: ati2dvag.dll
Driver Version: 6.14.0010.6860 | 8/20/2008 20:18:07
Disp Settings: Plug and Play Monitor | 1280 x 1024 (32 bit) (60Hz)

Audio Information
Audio Device: Auzen X-Fi Audio [EC00]
Driver Name: ctaud2k.sys
Driver Version: 6.00.0001.1281 | 8/22/2008 01:46:50
Default Sound Playback: true
Default Voice Playback: true

Any help is appreciated.