Some people are not aware that in older PCs the memory card readers will only read SD cards not SDHC (High Capacity cards). Most 4GB and over are SDHC cards. In order to read the data on those type of cards you will need a SDHC Reader.
One good SDHC reader that I have used is a MobileMate SD+. It has will read SDHC , miniSDHC and microSDHC cards. I picked mine up for about $14. Definiatly worth it to read those larger SD cards from your camera or phone.
Hope this helps.