When you go to 'My Computer', at the moment, you'll probably see something like these:
3� Floppy (A:)
Local Disk (C:)
Compact Disc (D:)
(Exactly what you see will depend upon what hardware your PC has got).
A card reader typically has 4 slots (to accommodate different types of cards). When you plug it into a USB port, your PC will automatically allocate the next 4 letters to the new drives. So now, you'll also see these drives:
Removable Disk (E:)
Removable Disk (F:)
Removable Disk (G:)
Removable Disk (H:)
(Each 'disk' simply represents one of the card slots).
The first time you use the card reader, you won't know which lettered drive is the one which corresponds for the slot with your card in it. For this first time only, it's a matter of trial and error. Double-click on Drive E. If you get a message saying 'Please insert a disk into drive E', you'll know that you've tried the wrong drive letter. Just click 'Cancel' and try Drive F, and so on.
When you get to the drive with your card in it, double-clicking on it will bring up the folder which contains your images. In future, you'll be able to go straight to the correct drive letter. (Either make a note of it or, if you want, right-click on the drive and use the 'Rename' option to call it something like 'Media Card').
Chris