Recently I've reformatted my pc and reinstalled Oblivion,

however it still powers off my computer

Originally, it usually occurred if I was in Sheogorath's throne room or Sithis's shrine

but since my reinstallation, it has happened 2 minutes after creating a character for a new game.

I have no mods installed except for, SI, all DLC but thieves den, the latest unofficial patches, and OBSE 21

Win7

8bg ram

Dx 11

i7-3517U cpu

Failing power supply, maybe?

Oblivion puts more load on your video card, so it draws more power. If your power supply is marginal, it may be overloading and shutting the system down.

Are you monitoring your CPU and GPU temperatures, and if you are, what are they running while you play?

It definitely sounds like a hardware problem.

Like other posters stated it does sound like hardware issue.

Do you have issues playing other games or using other programs.

Impossible to tell if its a hardware or software issue, at this point.

First does it shut down the computer or reboot/restart the system? If its restarting, it could be a BSoD with the automatically restart flag enabled. BSoD indicates multiple possible issues: faulty RAM, bad device driver, faulty GPU/CPU/Motherboard, etc. If it shuts down the computer (no restart) then it could be heat related or a power supply issue.

You may be able to determine a heat issue from the Windows Event Viewer, usually it will throw a warning message from the GPU/CPU.