can't access nas login page or shared files from win7 via wifi

My problem started when I upgraded from XP to Win7 on one of my computers.
Internet works fine, and I can see my nas (DNS-323) on my network places. I even can open the dns-323 and see the shares.

The problem starts when I open a folder on the nas containing any files, then the explorer hangs, nothing happens. I can browse the nas as long as its only folders showing up. Whenever there are files in it, it hangs and I have to force the window to close.
I also can't access the login page on the nas, it won't show up.

Now the funny part , the problems only appears when I use wlan, when I connect with a wire everythings works!!??

The computer is a LG netbook x120 with a built in realtek 8187se wifi nic and I upgraded to the latest driver 6.9109.
I also tested deactivate the built in wlan and pluged in a USB wlan nic, same problem.
Turned of windows firewall, same problem.
Turned of Avast antivirus, same problem.

Have anyone experienced the same problem or something similar?
I realy could need some help with this!

Sure there must be someone with a similar problem???
I think it can have something with my dsl-router but I'm not sure. It is a Speedtouch ST585 and in that case it should be some difference how it handle wireless and wired data on the internal side. I can't find any difference in the settings.

Any suggestion or input?

@ Henke:
Did you ever find a fix for your problem. I also have a ST585 and experience the exact same problem as you are describing and only from my netbook with Win7 Starter. I am thinking that it could somehow be related to the firmware of the router but have not tried to upgrade it yet.