My cardreader has always worked. When I put a card into the memory card slot, I have to go to Start -> My Computer and then double click on Removeable disk (G:).
But it depends which card you have. So try the different Removeable disks and see if one works.
If there are no Removeable disks in your "My Computer", then in the notification area (down the bottom left hand corner, there should be a little green arrow thing which says Safely Remove Hardware when you hover over it. Double click on this, and then there should be a component which is called "USB Mass Storage Device". If it is not there, then reinstall your memory card reader drivers.