apparently I seem to have solved this one. there are actually three possible solutions, I'm not sure which one really did the trick. one of them should be okay to be eliminated tho.. since its been working with that way before :)
1) removing of smaller RAM-stick
2) defragment (this is what I actually guess)
3) reducing multiple IP's on the same card to a single one.

number 3 is unlikely since its been working with that way before too. and, it didnt start to crash immediatly after I put them :) and number one isnt so likely because memtest reports the same errors with the other stick anyway. and, again, comp didnt start to crash immediatly after I reinstalled windows..
which, in turn, lets my to think that somehow the windows started to crash because something got corrupted/fragmented a bit too much or something. this makes sence in a way, since last time reinstallations of windows actually helped - which would most likely have more compact files. and with time, it started to fragment.. and MFT getting maybe too big and getting fragmented too, which might be something that then caused the crashings.
now, I'm interested to find out.. does this make sense to you ? :)