When you map the share check the box for remembering credentials.
"That dos ot help. Every time i start up i have to manualy enter my password"
"The second thing is that when i browse through the data with the explorer it is very slow. If i want to open a folder, i get a loop indicating that the explorer is searching, after 5 till 30 seconden the folder is opened and i see the data in it. With windows XP there is no delay."
This is likely due to the computer not the NAS. Do you have any other fileservers/NAS devices?
"No the NAS is my only file server. The windows XP machine and the windows 7 machine are in the same network"