You likely have an Atheros card and are using a Broadcom Driver for a different wireless card (which won't work). Try the latest Dell Atheros driver:

http://downloads.dell.com/FOLDER02256307M/1/3340_Network_Driver_19N7G_WN_10.0.0.306_A00.EXE

If that fails to install, provide Hardware IDs:

http://dellwindowsreinstallationguide.com/downloading-drivers-checking-hardware-ids-and-downloading-and-installing-dell-system-drivers-in-the-correct-order/checking-hardware-ids-in-the-device-manager/