The usual stuff is all I can suggest, I'm afraid...
Check the laptop with a wireless pcmcia card if you can, make sure it's not the windows wireless system screwing up.
Make sure your machine is service packed. Always helps.
Try a different router if you can (One that you can control... So not your neighbour's )
Try a windows system restore (the one that puts it back to a certain date). It may sound like a noob-ish solution, but it's got me out of a lot of problems.
Try the laptop with a knoppix bootdisk or something simaler to REALLY check it's not a windows problem.
Check your BIOS settings to make sure nothing has become disabled that shouldn't be (I'm not familiar with the BIOS setup of your laptop, so I'm not sure if there is an 'enable wireless' option somewhere)
If none of the above help... You might have to send your laptop back to dell. You could open it up and play about with it if you REALLY want, but it's not something I'd suggest on a 3 month old laptop.
Good luck :)
