I uninstalled all the previous NVIDIA drivers still left on my pc before installing the old one.
Tried the clean install now (with the newest driver), which had no effect at all.
Also my pc has no issue with any other game, just with DA:O so I don't think it is my cpu overheating. (doesn't that also restart your entire pc? I have had that happen before when the cooling got loose a little bit)
The 3D settings were on "Let the 3D application decide", so don't think there is an issue there.
When I lower the video settings to craptastic I haven't seen any crashes yet, even when I turn them up a bit (1680x1050, medium graphic detail, no AA, medium textures) it seems to be OK, but that is only over the course of about 10 minutes, and the game just looks so ugly then compared to how I could run it over a year ago.
I am playing the Darkspawn Chronicles so that is non stop fighting with a lot of NPC's etc. but that isn't extremely uncommon in DA:O.
As mentioned above I use the newest drivers 266.58 with the install, because the driver switching didn't seem to have any effect and my other games (i.e. World of Warcraft: Cataclysm) aren't really happy with the old junk. I can ofcourse switch back to the 196.21 drivers if you think that might help now.
Thanks for trying to help me out

The moments I actually can play the game I am enjoying it again and I can't wait to be able to properly play it once again.