PC restarts when playing games.

I'm experiencing a problem on windows7. I installed Windows7 last 2 months and until now, I haven't seem to have had any problem. My problem is that, when I load a game (Ragnarok) or watch a movie in my computer, it restarts. and no, I do not get the BSoD. It ran fine until this week. The only notable thing I did on my computer this week is that I changed my antivirus program and installed a new one. I have tried using System Restore and I still have the same problem so I've eliminated that as my problem. I'm thinking maybe it's the video card? but hell, I was able to run the game or the movies for 2months, why would it crash now.. and Ragnarok is a pretty lowtech game..

MY PC specs:
AMD Athlon 64 X2 3600+
2GB RAM
ATI Radeon X1200 Series 128MB



All help will be greatly appreciated.

@ttran
Done it. It didn't find any problem..

@zzz2496

Yes. by Gravity.

@zzz2496

I play on a private server but it didn't have any update prior to when the problem started so I doubt it's caused by some files, but care to point out which files you deleted?

@ttran

When it restarts, It just goes black, then immediately goes ON again, and at the startup, the Choose between Safe Modes and Regular Mode appears stating that it was shutdown to protect data and etc.
I monitor the temperatures while playing using HWMonitor by CPUID and everything seems to be fine..
Also, I don't have any other Online Game to test on, but it also occurs when I watch a movie..

I viewed the Event Viewer. Seems the source is Kernel-Power - EVENTID: 41 - TASK CATEGORY (61).

Viewed Details:

- System

- Provider

[ Name] Microsoft-Windows-Kernel-Power
[ Guid] {331C3B3A-2005-44C2-AC5E-77220C37D6B4}

EventID 41

Version 2

Level 1

Task 63

Opcode 0

Keywords 0x8000000000000002

- TimeCreated

[ SystemTime] 2010-01-03T04:54:22.519629700Z

EventRecordID 72404

Correlation

- Execution

[ ProcessID] 4
[ ThreadID] 8

Channel System

Computer Too - PC

- Security

[ UserID] S-1-5-18


- EventData

BugcheckCode 0
BugcheckParameter1 0x0
BugcheckParameter2 0x0
BugcheckParameter3 0x0
BugcheckParameter4 0x0
SleepInProgress false
PowerButtonTimestamp 0

So far, I've only updated the graphic driver. I don't know how to do BIOS, sound cards. Please guide me through a procedure.

Installing the DirectX End-User Runtime now.