im having the same problem and i tried the above comment ..did not work
Maybe you should try to isolate the problem. Go to your router/WLAN box and turn off all encryption and authorisation. Don't do your internet banking during this time if such things worry you.
If you can, check that a laptop or some other device connects with no authorisation.
Now, can the archos connect ?
Normally, you would not expect this to be the problem, since WEP/WPA/WPA2 are so standardised and work for so many people.
Turn security back on when you are finished experimenting.
I take it, you have no problems with signal strength.