Computer Reboots Every Hour

My computer reboots its self every hour, on the hour. The first time I had this issue, I re-installed Windows 7 64bit because I wasn't sure of the copy given to me. It worked great for a few months and yesterday it started to do it again. I have sleep mode turned off, display will never turn off, also I have it so it doesn't automatically reboot, but it is almost as if I'm holding the power button to shut it off. The only warning I get is the power supply will rev up and just restart. If I could get any help, it would be greatly appreciated.

Critical    4/5/2011 8:51:27 AM    Kernel-Power    41    (63)
Critical    4/5/2011 7:51:37 AM    Kernel-Power    41    (63)
Critical    4/5/2011 6:51:19 AM    Kernel-Power    41    (63)
Critical    4/5/2011 5:51:02 AM    Kernel-Power    41    (63)
Critical    4/5/2011 4:50:45 AM    Kernel-Power    41    (63)
Critical    4/5/2011 3:50:27 AM    Kernel-Power    41    (63)
Critical    4/5/2011 2:50:08 AM    Kernel-Power    41    (63)
Critical    4/5/2011 1:49:50 AM    Kernel-Power    41    (63)
Critical    4/5/2011 12:49:32 AM    Kernel-Power    41    (63)
Critical    4/4/2011 11:49:15 PM    Kernel-Power    41    (63)
Critical    4/4/2011 10:48:58 PM    Kernel-Power    41    (63)
Critical    4/4/2011 9:48:39 PM    Kernel-Power    41    (63)
Critical    4/4/2011 8:48:21 PM    Kernel-Power    41    (63)
Critical    4/4/2011 7:48:01 PM    Kernel-Power    41    (63)
Critical    4/4/2011 6:47:44 PM    Kernel-Power    41    (63)
Critical    4/4/2011 5:47:26 PM    Kernel-Power    41    (63)
Critical    4/4/2011 4:47:08 PM    Kernel-Power    41    (63)
Critical    4/4/2011 3:46:46 PM    Kernel-Power    41    (63)
Critical    4/4/2011 2:46:33 PM    Kernel-Power    41    (63)
Critical    4/4/2011 1:46:16 PM    Kernel-Power    41    (63)
Critical    4/4/2011 12:45:58 PM    Kernel-Power    41    (63)
Critical    4/4/2011 11:45:44 AM    Kernel-Power    41    (63)

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Log Name:      System
Source:        Microsoft-Windows-Kernel-Power
Date:          4/5/2011 8:51:27 AM
Event ID:      41
Task Category: (63)
Level:         Critical
Keywords:      (2)
User:          SYSTEM
Computer:      Tabby-PC
Description:
The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.
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>41</EventID>
    <Version>2</Version>
    <Level>1</Level>
    <Task>63</Task>
    <Opcode>0</Opcode>
    <Keywords>0x8000000000000002</Keywords>
    <TimeCreated SystemTime="2011-04-05T12:51:27.792821500Z" />
    <EventRecordID>8237</EventRecordID>
    <Correlation />
    <Execution ProcessID="4" ThreadID="8" />
    <Channel>System</Channel>
    <Computer>Tabby-PC</Computer>
    <Security UserID="S-1-5-18" />
  </System>
  <EventData>
    <Data Name="BugcheckCode">0</Data>
    <Data Name="BugcheckParameter1">0x0</Data>
    <Data Name="BugcheckParameter2">0x0</Data>
    <Data Name="BugcheckParameter3">0x0</Data>
    <Data Name="BugcheckParameter4">0x0</Data>
    <Data Name="SleepInProgress">false</Data>
    <Data Name="PowerButtonTimestamp">0</Data>
  </EventData>
</Event>