Problem is solved:

In the summer I've bought a new SSD (Samsung Series 830 - 128GB) because the 60GB I had became too small, and a new motherboard, because I wanted AHCI and sata600. Besides, I was hoping these replacements would solve the problem. Well.. they didn't.

Two weeks ago I found a nice deal for 16GB of RAM, which I bought.. And this seems to have solved my problem of endless blue screens and crashes. Strange, because I had tested the RAM for errors.

Anyhow, thank you all for your help. Happy that I can trust my machine again!