For anyone wanting to know how to "roll back" a driver as pointed out by Wildwood above...
1. Go into Control Panel
2. Select "Device Manager"
(alternatively, right click on Computer, then click Properties. A link for Device manager is now in the top left area of the window)
3. In device manager look for your wifi adapter (in the Network adapters section)
4. Right click on the wifi adapter and then click on Properties.
5. Select the Driver tab along the top
6. If the driver has been updated recently, you'll be able to select the "Roll Back Driver" button
For the record, this happens with other devices too such as sound, graphics, camera's etc.