Recently my home built computer has been rebooting itself during games such as the new Sim City and Anno 2070. I have replaced my graphics card with a GT 620 as my old card wasn't up to the graphics, but with the old card my PC was still rebooting.  I checked Event Viewer and the following was listed as critical -

Log Name: System
Source: Microsoft-Windows-Kernel-Power
Date: 24/03/2013 8:56:21 p.m.
Event ID: 41
Task Category: (63)
Level: Critical
Keywords: (2)
User: SYSTEM
Computer: ******
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>3</Version>
<Level>1</Level>
<Task>63</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000002</Keywords>
<TimeCreated SystemTime="2013-03-24T07:56:21.510352600Z" />
<EventRecordID>22567</EventRecordID>
<Correlation />
<Execution ProcessID="4" ThreadID="8" />
<Channel>System</Channel>
<Computer>Soprano</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">0</Data>
<Data Name="PowerButtonTimestamp">0</Data>
<Data Name="BootAppStatus">0</Data>
</EventData>
</Event>

This has happened after installing a new power supply today which is now a Corsair VS550 (550) watt compared to my old 460 watt. 

Any ideas on what could be causing this as its beyond annoying now 

Below are specs as per Speccy software if this is any use. All drivers are up to date too. 

Operating System
MS Windows 8 32-bit
CPU
AMD Athlon 64 X2 6000+ 46 °C
Windsor 90nm Technology
RAM
4.00 GB Dual-Channel DDR2 @ 334MHz (5-5-5-15)
Motherboard
Gigabyte Technology Co., Ltd. GA-MA770-DS3 (Socket M2) 29 °C
Graphics
L226WTP (1680x1050@60Hz)
Microsoft Basic Render Driver
NVIDIA GeForce GT 620
Hard Drives
488GB SAMSUNG SAMSUNG HD501LJ ATA Device (SATA) 24 °C
Optical Drives
ASUS DRW-2014L1T ATA Device
Audio
High Definition Audio Device