Windows 7 keeps asking for WIFI credentials

Question

If I want to log onto our WIFI network, Windows keeps asking for credentials. After many connection attempts (and sometimes some reboots) we finally manage to connect. Then the connection usually stays intact for the rest of the day. But it sometimes takes 15-20 minutes before we're online... Needless to say we're using a single username/password for all failing/succeeding attempts.

Does anybody recognize these symptoms? How to troubleshoot this?

UPDATE 1

The problem is somewhat complicated by our Windows being a Dutch version (not my choice). The window that keeps popping up is the following (perhaps I can edit it in order to translate...).

Answer 1

You need to delete information about WiFi-network in "Network and Sharing Center" --> "Wireless Networks". After this you need to reconnect again.

Where are you using username/password in WiFi connection?

Try to play with switch "IEEE 802.1x authentication" for this network http://windows.microsoft.com/en-us/windows-vista/enable-802-1x-authentication

Answer 2

I had WEP in my router and someone using win8.1 started to see the same issue (It was working in the past without asking for username and password). I didn't find any solution. I ended up changing my router WEP to WPA2+PSk and that solved the issue