Secured network properties

Question

I used to be prompted by my Acer computer for the passphrase for a network but somehow the settings of the properties must have changed for encryption type and security type and now the computer is trying to connect without asking me for a passphrase (password). How do I reset the properties that may have changed and to what so that the Acer asks me for a passphrase again? I think the security type is wep from observing by Toshiba laptop. I never connected to this network because I cannot get the passphrase for this secured network and I don't wish to. I have windows vista home premium. I have an Acer Extensa 5420-5038 laptop.