Windows 7 x64 machine suddenly performs restart as if I hit the button

My HTPC has restarted itself probably 2-3 times in the last week. It's your standard Windows 7 restart complete with closing all apps, Logging Off, and Shutting Down screens. The most recent occurrence was just a few minutes ago. I was playing a video, running ATI CCC testing settings, and had Chrome open. I never issued this command or touched the PC itself. One thing I know was also going on was the daily MSE security definitions update, most likely in the install phase. I do not recall what was going on in the the previous times this happened, but it's all pretty recent.

This one has me stumped since it's not a blue screen, but a controlled restart with no errors or problems indicated. I can't for the life of me think of what could cause such a thing to happen. Can anyone think of any possible sources of a Restart command? Is MSE issuing this after definition updates? Is there a way to check the source of a restart command like in a windows system log?

EDIT: in Event Logs I found this:

Code:

Log Name:      System
Source:        Microsoft-Windows-Kernel-Power
Date:          10/22/2013 6:05:32 PM
Event ID:      109
Task Category: (103)
Level:         Information
Keywords:      (4)
User:          N/A
Computer:      Tower_Of_Power_
Description:
The kernel power manager has initiated a shutdown transition.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Microsoft-Windows-Kernel-Power" Guid="{331C3B3A-2005-44C2-AC5E-77220C37D6B4}" />
    <EventID>109</EventID>
    <Version>0</Version>
    <Level>4</Level>
    <Task>103</Task>
    <Opcode>0</Opcode>
    <Keywords>0x8000000000000004</Keywords>
    <TimeCreated SystemTime="2013-10-22T23:05:32.272660300Z" />
    <EventRecordID>85572</EventRecordID>
    <Correlation />
    <Execution ProcessID="532" ThreadID="536" />
    <Channel>System</Channel>
    <Computer>Tower_Of_Power_</Computer>
    <Security />
  </System>
  <EventData>
    <Data Name="ShutdownActionType">5</Data>
    <Data Name="ShutdownEventCode">0</Data>
    <Data Name="ShutdownReason">5</Data>
  </EventData>
</Event>

And also this. Clearly here is the problem but why is it happening and what does it mean?

Code:

Log Name:      System
Source:        USER32
Date:          10/22/2013 6:05:06 PM
Event ID:      1074
Task Category: None
Level:         Information
Keywords:      Classic
User:          SYSTEM
Computer:      Tower_Of_Power_
Description:
The process C:\Program Files (x86)\Malwarebytes' Anti-Malware\mbam.exe (TOWER_OF_POWER_) has initiated the restart of computer TOWER_OF_POWER_ on behalf of user NT AUTHORITY\SYSTEM for the following reason: Legacy API shutdown
 Reason Code: 0x80070000
 Shutdown Type: restart
 Comment: 
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="USER32" />
    <EventID Qualifiers="32768">1074</EventID>
    <Level>4</Level>
    <Task>0</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2013-10-22T23:05:06.000000000Z" />
    <EventRecordID>85524</EventRecordID>
    <Channel>System</Channel>
    <Computer>Tower_Of_Power_</Computer>
    <Security UserID="S-1-5-18" />
  </System>
  <EventData>
    <Data>C:\Program Files (x86)\Malwarebytes' Anti-Malware\mbam.exe (TOWER_OF_POWER_)</Data>
    <Data>TOWER_OF_POWER_</Data>
    <Data>Legacy API shutdown</Data>
    <Data>0x80070000</Data>
    <Data>restart</Data>
    <Data>
    </Data>
    <Data>NT AUTHORITY\SYSTEM</Data>
    <Binary>00000780000000000000000000000000000000000000000000000000000000000000000000000000</Binary>
  </EventData>
</Event>

Ok so I've found the issue. Event Logs showed MalwareBytes restarting after a daily quick scan and removal of suspicious software:

Code:

Malwarebytes Anti-Malware (PRO) 1.75.0.1300
www.malwarebytes.org

Database version: v2013.10.22.10

Windows 7 Service Pack 1 x64 NTFS
Internet Explorer 10.0.9200.16721
SYSTEM :: TOWER_OF_POWER_ [limited]

Protection: Enabled

10/22/2013 6:00:12 PM
mbam-log-2013-10-22 (18-00-12).txt

Scan type: Quick scan
Scan options enabled: Memory | Startup | Registry | File System | Heuristics/Extra | Heuristics/Shuriken | PUP | PUM | P2P
Scan options disabled: 
Objects scanned: 221031
Time elapsed: 4 minute(s), 54 second(s)

Memory Processes Detected: 0
(No malicious items detected)

Memory Modules Detected: 0
(No malicious items detected)

Registry Keys Detected: 0
(No malicious items detected)

Registry Values Detected: 0
(No malicious items detected)

Registry Data Items Detected: 0
(No malicious items detected)

Folders Detected: 0
(No malicious items detected)

Files Detected: 1
C:\Users\Trunk Monkey\Downloads\attsetup.exe (PUP.Optional.OpenCandy) -> Quarantined and deleted successfully.

(end)

About an hour ago I downloaded ATI Tray Tools from Softpedia. MBAM flagged and deleted the installer, which had not been run yet. I wonder if it's a false positive. Hmmmm. Posting all this in case someone else ever encounters mysterious restarts. I'll update anything new I find out.

thanks for the advice, i'll take heed. I thought Softpedia was one of the good ones. But I guess it's installer beware. Thankfully I never installed it.