My computer randomly switches off, no BSOD.

I had upgraded my system from a GTX 420 (OEM) to a GTX 460 last month, I had problems with my sound but I reinstalled windows and everything was working fine until I tried to run the game magicka it crashed and terrible things happened my computer rebooted a few times. I restored my computer to a previous date everything seemed to be working fine but then while playing another game (TF2) it switched off but all the fans and lights where still on. It has done this a few times now with just internet browsing and other non intensive things.

It was a pre-built Packard bell ixtreme I5726 UK

Welcome to the forum,

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I had a similar problem when I upgraded my GPU to a 8800GT, turned out the PSU was not good enough, even though it was a 550 PSU, it was a crappy unit that came with the box, in the end I chose a quality PSU, a Corsair HW520, that was 4 years ago and I am now using it with a GTX 560, and NEVER had a problem once I installed a quality PSU, hope it is not your problem as you would have to spend more money. (I am upgrading my PSU to a Corsair AX850 for future proofing).

I was thinking it may be that but I there is nothing actually wrong with the psu (got the multimeter out and checked all the cables). But are you saying that the psu can't keep up.

I did #3