I recommend booting the computer to Safemode with networking (Restart the computer and tap F8 on Dell logo and select Safemode with networking).
Check if wireless works fine is Safemode with networking.
If the wireless shows the same behavior, update wireless drivers from below link:
Visit the link: http://dell.to/QjjZDA
Now try connecting to wireless network and check if that works.
For more information, I have provided a link below which can fix the issue:
<ADMIN NOTE: Broken link has been removed / replaced from this post by Dell>
Below mentioned link would also help you with the settings for wireless internet:
Please reply if you have any further questions.