OK.. looks like Nikhil had you do everything except uninstalling the driver first? Perhaps that is worth an attempt. Go to your Device Manager and right click the USB Host Controller that has the 'Exclamation point' select the Uninstall option from the drop down. Let it completely uninstall.. then reboot your system.. Now you can try two different approaches.. one use Windows update to see if it finds another driver to use or use the Dell links provided by Nikhil and install them from Dell's site again.. See if that helps. It may detect the hardware during the boot sequence and ask to install the driver again, if this happens let it :-)
Reason: You could have a corrupted driver that is so badly corrupted that just upgrading it might not work..
Let us know .. I'm thinking if this doesn't work call Dell Tech support see if they can help, if not you might simply have a bad Controller or the SD Slot could have issues directly.
Depending when you got your system, you might still be under warrantee if you've extended your warrantee this should be covered as it sounds like a manufacturer's defect :-) =Possibly.. not saying it is yet.. Nikhil has a point about details and troubleshooting before you say it's a defect.