With you using System Restore to when it worked or having Dell repair the machine.
My Dell used to detect all wireless networks properly. However, just recently, it has stopped detecting networks when I try to connect. My router is setup properly as others are able to connect, just not me.
I am running Vista, but was also running Vista when it used to work. My wifi is enabled.
What is weird is that when I go to Dell Wireless WLAN Card Utility, it detects my network but nothing shows when I try Windows Connect to a Network.