Opened the "Dell Control Point" software.

Selected "Connection Manager".

Clicked on the "Connections" button.

Selected "Profiles".

Selected "Change Settings".

Selected "Program Settings".

This displayed a dialog with a group named "Available technologies" that contains 5 check boxes (Ethernet, Wi-Fi, Mobile Broadband, Dial Up and GPS).  I then deselected the Wi-Fi check box.

This then disconnected me from the internet (as expected) but then when I opened the WLAN card utility software the option to "Let this tool manage your wireless networks" was enabled and I could successfully add (and manage) my wireless network using the WLAN utility.  Hurrah!

(see full article at http://en.community.dell.com/support-forums/network-internet-wireless/f/3324/t/19252153.aspx)