You have to trouble shoot the problem it self.
A few things to try.
Move closer to see if it is better.
Try to change the wireless channel in the router.
Try to connect to a friends network(to rule out the router as an issue).
If you have been inside the notebook, please make sure you have not disconnected or or more of the antennas
You may try to let Windows search Windows Update for a newer driver than what Dell is offering
You have many things that can go wrong and a few steps to try to track down the issue.
In rare cases some wireless hardware just does not work well together.
EDIT again.
If it worked fine on Windows 7, you may be able to use the same drive from Windows 7 on Windows 8.1 to see if it is any better. Did you take note of the driver version under Windows 7?