Okay I have checked some more and it has definitly to do something with the fast user switching (http://forums.dlink.com/index.php?topic=3554.0).. I can change the interrupt setting on the other account on the laptop but it will flip back automaticly... argh!
I have managed to fix it by deleting the printer on the vista laptop. I connected the printer directly to the laptop, installed the drivers, and connected the printer back to the router. I hope this will last.. if not, I'll post here.