Computer Powers clicks off without warning

Lately my computer's power turns off without warning. I'm not a complete computer newbie, I've built this system and I know most of my basic stuff. First off, I'll list what I've ruled out. Heat, no. Its not overheating. I use a system monitor and an on board thermometer that never reaches 50C. I've also tested my power supply, volts/watts are all running at normal levels. I've tested my RAM chips, rearranging them and such, but thorough testing of pairs and individuals have afforded any different results (so I'm not sure about that).
What its doing. First off, i've had a drastic increase in loading time to boot windows starting from the logo. What once took less than a minute now takes about 10 literal minutes. I've noticed on occasion my cpu working very hard over nothing (no additional tasks in processes). And that darndable power off that comes almost without reason. I have notice that the seemingly random poweroff during boot occurs almost always during the DMI pool data test. However, it occurs often during the logo boot, and after selecting a user to log on to. I've also noticed that it tends to power off when I put it to work. However, it also powers off when its sitting idle and doing nothing. Click-dead. Simple as that. Any ideas? I'm more hardware knowledgeable than software, so you'll have to step me through software related tests.

This might help.

System Information

Time of this report: 10/20/2012, 09:37:18
Machine name: LACKEYS-F9E0670
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 3 (2600.xpsp_sp3_gdr.120504-1619)
Language: English (Regional Setting: English)
System Manufacturer: NVIDIA
System Model: AWRDACPI
BIOS: Phoenix - AwardBIOS v6.00PG
Processor: AMD Athlon(tm) 64 X2 Dual Core Processor 3800+, MMX, 3DNow (2 CPUs), ~2.0GHz
Memory: 3200MB RAM
Page File: 689MB used, 7348MB available
Windows Dir: C:\WINDOWS
DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
DxDiag Version: 5.03.2600.5512 32bit Unicode
DxDiag Previously: Crashed in system information (stage 4)

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DxDiag Notes
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DirectX Files Tab: No problems found.
Display Tab 1: No problems found.
Sound Tab 1: No problems found.
Sound Tab 2: No problems found.
Music Tab: No problems found.
Input Tab: No problems found.
Network Tab: No problems found.

I have the full dxdiag file if anyone needs to see any specific part. Thanks in advance.