Demarrage de mon pc Nec powermate vl370

Salut j'ai eu recemment un pc de marque NEC powermate _vl370 . En effet quand je démarre ce PC j'entends le ventilo tourner très vite et lorsque j'appuie sur le bouton de marche/arrêt rien ne se passe. Ce qui me surprends c'est lorsque je débranche et rebranche le câble d'alimentation de l'UC et ooop le pc démarre sans que j'appui la touche de marche /arrêt et tout se passe normalement. Est ce normal? sinon quel est le problème? Toutes les composantes sont bien branchées à la carte mère.Voici un examen dxdiag du pc.
System Information
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Time of this report: 11/2/2015, 17:41:11
Machine name: MOHAMED-PC
Operating System: Windows 7 Édition Intégrale 32-bit (6.1, Build 7600) (7600.win7_rtm.090713-1255)
Language: French (Regional Setting: French)
System Manufacturer: NEC COMPUTERS SAS
System Model: POWERMATE_VL370
BIOS: )Phoenix - Award WorkstationBIOS v6.00PG
Processor: AMD Athlon(tm) Dual Core Processor 4450e (2 CPUs), ~2.3GHz
Memory: 2048MB RAM
Available OS Memory: 1918MB RAM
Page File: 2150MB used, 1686MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
DxDiag Version: 6.01.7600.16385 32bit Unicode

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DxDiag Notes
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Display Tab 1: No problems found.
Sound Tab 1: No problems found.
Input Tab: No problems found.

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DirectX Debug Levels
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Direct3D: 0/4 (retail)
DirectDraw: 0/4 (retail)
DirectInput: 0/5 (retail)
DirectMusic: 0/5 (retail)
DirectPlay: 0/9 (retail)
DirectSound: 0/5 (retail)
DirectShow: 0/6 (retail)

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Display Devices
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Card name: ATI Radeon X1200 Series
Manufacturer: ATI Technologies Inc.
Chip type: ATI Radeon X1200 Series (0x791E)
DAC type: Internal DAC(400MHz)
Device Key: Enum\PCI\VEN_1002&DEV_791E&SUBSYS_E9021631&REV_00
Display Memory: 831 MB
Dedicated Memory: 127 MB
Shared Memory: 703 MB
Current Mode: 1280 x 1024 (32 bit) (60Hz)
Monitor Name: Moniteur Plug-and-Play générique
Monitor Model: L1919S
Monitor Id: GSM4AF0
Native Mode: 1280 x 1024(p) (60.020Hz)
Output Type: HD15
Driver Name: atiumdag.dll,atiumdva.cap,atitmmxx.dll
Driver File Version: 8.14.0010.0647 (English)
Driver Version: 8.593.100.0
DDI Version: 9Ex
Driver Model: WDDM 1.0
Driver Attributes: Final Retail
Driver Date/Size: 2/11/2010 06:17:14, 3839488 bytes
WHQL Logo'd: n/a
WHQL Date Stamp: n/a
Device Identifier: {D7B71EE2-3A5E-11CF-0677-08C9A1C2C535}
Vendor ID: 0x1002
Device ID: 0x791E
SubSys ID: 0xE9021631
Revision ID: 0x0000
Driver Strong Name: oem1.inf:ATI.Mfg.NTx86.6.0:ati2mtag_RS690:8.593.100.0:pci\ven_1002&dev_791e
Rank Of Driver: 00F62001
Video Accel: ModeMPEG2_A ModeMPEG2_C