Hopefully it might point towards the culprit.
I had this type of problem twice before.
Depending on how heavy my usage was at the time, sometimes it would go for a couple of days before the BSOD or re-start.
My problem turned out to be a faulty fan on the graphics card.
(But yes it could also be the fan inside the PSU)
Once I changed the graphics card there was no problem for a long time, but it then re-occurred so this time I changed only the fan, which was obtained from Maplin Electronics and it has now been working fine for the last 2 years
If you open up the PC case and see if the fan rotates fairly easily.
The warmer the card gets then the more likely it is that the fan stops working, thus creating a shut-down of the PC.