When you downloaded the drivers from Dell did a program get installed to manage your network connections?
My brother and sister-in-law had a problem similar when I recently fixed their laptop. Basically they have Charter broadband internet, and so instead of using Charter's drivers for the device, I got the regular drivers off the manufacturer's website. When I installed it, I told it to use Windows XP for managing the wireless connections, but when they shut it down it reverted back to the manufacturer's program, and they lost the connection.
But I think after I changed it back the second time it hasn't done it since.