You are trying to download the photos from the digital camera, the photos are very important to you but your SD card cannot be read it due to some reason and you are in a real problem. Before directly diving into the problem itself it is better to understand the different ways of photo transfer and how to use different utilities provided by the operating system of your computer to do it.
The different ways:--
1) Directly connect the camera to one of the USB ports of the computer and then turn the camera on (it is better way of doing it instead of hot plugging the already turned on camera to the computer). The "My Computer" for Windows XP or only "Computer" for Vista should be able to show the camera as one of the drives. You only need to open the camera as a drive and copy paste or cut paste it one of the folders on your computer. You need proper softwares to open those photos in your computer.
2) Use a separate memory card reader to read the data in the card. Connect the card reader to the computer and that should again get detected as a drive in the "My Computer".
3) The third way is to directly print out the photos using a "Pict-bridge" which is a slot in your printer where you can insert the memory card and you should be able to see the photos in the printer display. People do face problems with this option.
We are considering the option one and two. Except trying to access the photos from the card as a drive in the computer, there are utilities as well like the Scanners and Camera wizard which should be selected as an option when a pop-up window appears in the middle of the screen as shown below.
You can also use third party softwares like HP Photosmart Essential (if you have a HP printer then you should have this software in your computer) to retrieve photos from cameras no matter if the brand of the camera is non-HP.
However, the card itself might not get detected and it would be impossible to read the card at times. You can try out the following:-
1) Check if it is more than 2 GB the reader should be compatible with it.
2) Before using the card make sure to format it as FAT 32
3) Does the card read if you put it in your phone? If yes then may be you have put a password to it and that is why the card reader is not reading it.
4) Get a better reader which can read high capacity cards. Also check whether the problem is with the reader not getting detected in the device manager or not. Also try a different computer. Take technical help to resolve driver issues.
5) Third party softwares are available for such stubborn data recovery although it is not guaranteed.
6) Give it a better push into the slot to eliminate a loose connection problem.