Thanks!
I'll ask my landlord for some help with this. Actually, I took the old memory with me to the shop to make sure I got a compatible one, but they did warn me that it might not work for the one is 256, a cheap one that was in the PC when I got it and the new one is 512. I'll try taking the old one out and working only with the new one for the moment.
Okay, I'll shut down and do this right now for this is really driving me nuts...
P.S.: Oh, sorry, hab gar nicht gemerkt, dass ich Englisch geschrieben hab. Hoffentlich kein Problem? Ich hab jetzt erst mal den 512er rausgenommen und es scheint zu gehen. Mal sehen und auf den Holzkopf klopfen...
Gruss,
Kobihofmann