Is the wireless driver loaded? The last two Dell machines I reloaded, I had trouble locating which actual wireless device was installed as the Dell software didn’t do a good job picking it up or installing the driver at all.
The driver is almost certainly your problem. Drove me crazy until I figured that out. The drivers are on the Dell utilities disk, but they are hard to find. As I recall, I found them in the files for the Dell Connection Manager. I had to install this application to get the drivers and then later uninstalled Dell Connection Manager which I found to be a very annoying little program.