But since you have ideas of your own I would suggest you test those.
Why not try a System Restore back to when it was working? This should uninstall the game and that .NET Framework. If that cures it, then you know. If not, you can always reinstall those, and then I would start looking for malware.
Update and scan with your anti-virus, and I would also download, install, update and scan with the free versions of Malwarebytes Anti-malware and SUPERAntiSpyware.
I have a Windows XP SP3 Home Edition computer.
768mb DDR Ram
40gb 7200 HDD
The computer "shutdown dialog"/"turn off computer window" keeps coming up at random times (the window where it says turnoff/standby/logoff. However it always comes up the first time when I turn on the computer. Then it comes up randomly while I am web browsing.
Could it be a virus? It was a recently installed Windows computer.
Could it be .NET Framework? I have recently installed it to play Terraria.
What could it be?