I first thought that it might be some DVD device thing but the problem persisted even after unplugging it. In december the machine already was at my vendor but they told me that it was a driver problem since everything ran fine on a fresh installed machine... so I reinstalled everything and the problem was still there.
I remarked that the machine ran fine when unplugging the second gig of RAM, so I ran it for 2 weeks with only one gig. I informed my hardware vendor about my observation and suggested to replace the entire RAMs because I thought about some kind of incompability between the RAM Dimms, which he rejected.
Now, I did some more tests and finally only put the second gig into my machine (thus having only on gig running). And the machine didn't work anymore. I couldn't boot up and had BSODs during startup. It didn't matter if I started normally or in secure mode. I put back in the first gig and everything ran. Put back the second gig (removing the first one) -> nothing works... so it's the RAM... something my vendor should have found when I gave him the machine the first time.
I informed my hardware vendor again and he told me to bring in the machine to check it again... So this morning I dropped him the machine and he told me that he would replace the RAM if he can reproduce the errors.
Thinking back it is obvious why the fresh install of the OS didn't cause any trouble: Windows doesn't use all the 2 gigs if you have a fresh install.
Hopefully I'll get my machine this evening or tomorrow during the day...