Must install this first:

http://www.dell.com/support/drivers/us/en/19/DriverDetails/Product/vostro-1440?driverId=CMR32&osCode=W764&fileId=2731102127&languageCode=EN&categoryId=CS

Then this

http://www.dell.com/support/drivers/us/en/19/DriverDetails/Product/vostro-1440?driverId=DHJ2D&osCode=W764&fileId=2993730947&languageCode=EN&categoryId=NI

Then this

http://www.dell.com/support/drivers/us/en/19/DriverDetails/Product/vostro-1440?driverId=CXW91&osCode=W764&fileId=2731102159&languageCode=EN&categoryId=NI

Firstly thank you so much for replying to this post but the method you showed me didn't work. I tried re-installing windows but didn't work as well. Those drivers are really 'stubborn'. Any other ways?

Actually when i tried manually installing the drivers, it appears that the third driver that you gave me is not supported by the hardware? I am not sure how to go on now. Should I send it to the shop?

Please provide the hardware IDs are there are multiple wireless card variants in each Dell system. For instructions see here:

http://dellwindowsreinstallationguide.com/downloading-drivers-checking-hardware-ids-and-downloading-and-installing-dell-system-drivers-in-the-correct-order/checking-hardware-ids-in-the-device-manager/

Also refer to driver installation order (I explain it in a bit more detail than the official Dell article):

http://dellwindowsreinstallationguide.com/downloading-drivers-checking-hardware-ids-and-downloading-and-installing-dell-system-drivers-in-the-correct-order/driver-installation/