Windows Crashing, no BSOD nor dump file

My Windows is crashing without a BSOD. It just shuts down. No message or anything.

I have tried looking at the Event Viewer to find what might be the cause of it, but I haven't found anything, just the errors. I'm not very knowledgeful on this, but I'll do anything you guys tell me to.

There appears to be no dump file(either on C:/Windows/minidump/ folder and there's no file C:/Windows/MEMORY.dmp file).

I'm attaching the folder created by SF Diagnostic Tool...

I appreciate the time you're taking in helping me out :) Thanks

Thank you very much. Here's the result:

Code:

Diagnostic Report (1.9.0027.0):
-----------------------------------------
Windows Validation Data-->

Validation Code: 0
Cached Online Validation Code: 0x0
Windows Product Key: *****-*****-GJY49-VJBQ7-HYRR2
Windows Product Key Hash: W5/6nm6F2UPXrCkY5xUhXb/+21g=
Windows Product ID: 00426-OEM-8992662-00006
Windows Product ID Type: 2
Windows License Type: OEM SLP
Windows OS version: 6.1.7601.2.00010100.1.0.001
ID: {23ECA6B0-A103-4CF9-8FA3-626725E28776}(1)
Is Admin: Yes
TestCab: 0x0
LegitcheckControl ActiveX: N/A, hr = 0x80070002
Signed By: N/A, hr = 0x80070002
Product Name: Windows 7 Ultimate
Architecture: 0x00000009
Build lab: 7601.win7sp1_gdr.130104-1431
TTS Error: 
Validation Diagnostic: 
Resolution Status: N/A

Vista WgaER Data-->
ThreatID(s): N/A, hr = 0x80070002
Version: N/A, hr = 0x80070002

Windows XP Notifications Data-->
Cached Result: N/A, hr = 0x80070002
File Exists: No
Version: N/A, hr = 0x80070002
WgaTray.exe Signed By: N/A, hr = 0x80070002
WgaLogon.dll Signed By: N/A, hr = 0x80070002

OGA Notifications Data-->
Cached Result: N/A, hr = 0x80070002
Version: N/A, hr = 0x80070002
OGAExec.exe Signed By: N/A, hr = 0x80070002
OGAAddin.dll Signed By: N/A, hr = 0x80070002

OGA Data-->
Office Status: 109 N/A
OGA Version: N/A, 0x80070002
Signed By: N/A, hr = 0x80070002
Office Diagnostics: 025D1FF3-364-80041010_025D1FF3-229-80041010_025D1FF3-230-1_025D1FF3-517-80040154_025D1FF3-237-80040154_025D1FF3-238-2_025D1FF3-244-80070002_025D1FF3-258-3

Browser Data-->
Proxy settings: N/A
User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Win32)
Default Browser: C:\Program Files (x86)\Mozilla Firefox\firefox.exe
Download signed ActiveX controls: Prompt
Download unsigned ActiveX controls: Disabled
Run ActiveX controls and plug-ins: Allowed
Initialize and script ActiveX controls not marked as safe: Disabled
Allow scripting of Internet Explorer Webbrowser control: Disabled
Active scripting: Allowed
Script ActiveX controls marked as safe for scripting: Allowed

File Scan Data-->

Other data-->
Office Details: <GenuineResults><MachineData><UGUID>{23ECA6B0-A103-4CF9-8FA3-626725E28776}</UGUID><Version>1.9.0027.0</Version><OS>6.1.7601.2.00010100.1.0.001</OS><Architecture>x64</Architecture><PKey>*****-*****-*****-*****-HYRR2</PKey><PID>00426-OEM-8992662-00006</PID><PIDType>2</PIDType><SID>S-1-5-21-3828357446-3422893705-3713493050</SID><SYSTEM><Manufacturer>BIOSTAR Group</Manufacturer><Model>NF61S-M2A</Model></SYSTEM><BIOS><Manufacturer>Phoenix Technologies, LTD</Manufacturer><Version>6.00 PG</Version><SMBIOSVersion major="2" minor="4"/><Date>20070208000000.000000+000</Date></BIOS><HWID>11520C00018400F8</HWID><UserLCID>0C0A</UserLCID><SystemLCID>0C0A</SystemLCID><TimeZone>Central Standard Time (Mexico)(GMT-06:00)</TimeZone><iJoin>0</iJoin><SBID><stat>3</stat><msppid></msppid><name></name><model></model></SBID><OEM><OEMID>ACRSYS</OEMID><OEMTableID>ACRPRDCT</OEMTableID></OEM><GANotification/></MachineData><Software><Office><Result>109</Result><Products/><Applications/></Office></Software></GenuineResults>  

Spsys.log Content: 0x80070002

Licensing Data-->
Software licensing service version: 6.1.7601.17514

Name: Windows(R) 7, Ultimate edition
Description: Windows Operating System - Windows(R) 7, OEM_SLP channel
Activation ID: 7cfd4696-69a9-4af7-af36-ff3d12b6b6c8
Application ID: 55c92734-d682-4d71-983e-d6ec3f16059f
Extended PID: 00426-00178-926-600006-02-3082-7601.0000-1882012
Installation ID: 017291571856416534921894765255071530931921021086400865
Processor Certificate URL: http://go.microsoft.com/fwlink/?LinkID=88338
Machine Certificate URL: http://go.microsoft.com/fwlink/?LinkID=88339
Use License URL: http://go.microsoft.com/fwlink/?LinkID=88341
Product Key Certificate URL: http://go.microsoft.com/fwlink/?LinkID=88340
Partial Product Key: HYRR2
License Status: Licensed
Remaining Windows rearm count: 4
Trusted time: 20/02/2013 16:12:04

Windows Activation Technologies-->
HrOffline: 0x00000000
HrOnline: N/A
HealthStatus: 0x0000000000000000
Event Time Stamp: N/A
ActiveX: Registered, Version: 7.1.7600.16395
Admin Service: Registered, Version: 7.1.7600.16395
HealthStatus Bitmask Output:


HWID Data-->
HWID Hash Current: LAAAAAAAAgABAAEAAQABAAAAAQABAAEAeqj0AUjRkgDIkhKjTH82xFKZyPQ=

OEM Activation 1.0 Data-->
N/A

OEM Activation 2.0 Data-->
BIOS valid for OA 2.0: yes
Windows marker version: 0x20001
OEMID and OEMTableID Consistent: yes
BIOS Information: 
  ACPI Table Name    OEMID Value    OEMTableID Value
  APIC            Nvidia        NVDAACPI
  FACP            Nvidia        NVDAACPI
  HPET            Nvidia        NVDAACPI
  MCFG            Nvidia        NVDAACPI
  SSDT            PTLTD         POWERNOW
  SLIC            ACRSYS        ACRPRDCT

No idea, I bought the whole pc from a friend. He sold it complete to me, is that why it's crashing? Did he sold me a fake pc? What can I do?

Oh, ok :P Thanks!