I suspect you may have either bad hardware or some compatibility problem. Does your old PC have USB 2 or 1.1? Older standards do have some compatibility issues with newer hardware and not all newer hardware is completely backward compatible. Have you tried the same reader and memory card in a newer system? I'd do that first and see if it works. If it does, you may need a different reader or maybe even a USB 2 add in card to make them work together properly.
I have a 6 year old, Dell Windows XP Professional computer. My HP printer has a memory card reader as part of it - the printer is about 4 years old. When I put an SDHC memory card in my HP memory card reader, a box appears on the screen asking me to put a disk in that drive (indicating the card is not recognized). An SDC card and other style cards such as Compact Flash cards are recognized by this reader.
When I put an SDHC into a USB memory card reader and attach it to my Windows XP computer, the memory card reader is slowly recognized by my computer but a copy and transfer of a file from the memory card to my hard drive never completes.
When I use an SDC instead of an SDC card, I have none of the problems mentioned above. All functions normally. I tried all of this with three different memory card readers with the same results. I also tried the memory card readers with an SDHC card on my newer Windows 7 computer. No problems using the SDHC card on my newer Windows 7 computer.
So, is it possible that my 6 year old computer just can't handle SDHC memory cards and can only handle the the older SDC memory cards? Does this make sense or could there be some other reason for my problem?