Do you know which wireless adapter you have installed in your laptop?  Looking at the drivers and download page for your system there are a couple different adapter drivers listed. And it is not entirely out of the question that the adapter in your sytem is not on your CD. I have run across this before. This could be a reason.  To find out which adapter you have if you are not 100%v positive you can go HERE enter your service tag and get a listing of the hardware that was shipped with your system. This list can be somewhat cryptic so if you are not sure how to read it you can come back and post it and we can help decypher it. Just make sure that if you do post it you do not post your service tag number or order number and such as that is a violation of the rules of this forum
I was wondering if anyone could help me with this problem I've been having with my laptop. Following the directions of a Dell technician while trying to resolve a separate problem, I deleted the partitions on my hard drive and manually reinstalled both Vista and all the drivers that were on the Dell CD that came with my laptop. When I tried connecting to the internet, Vista said I did not have any wireless networking hardware, but before the reinstallation it worked just fine. I am able to connect to the internet using a wired connection, but my wireless adapter still isn't working. I use Windows Vista 32-bit SP1, if that helps anyone.
XPS M1530 Does Not Recognize Wireless Network Adapter after Maunal 
Reinstallation of Vista and Dell Drivers