Is a 3G USB internet device compatible with a 5 year old desktop PC?

I switched from a broadband connection with a wall cable to a USB card because Sprint said it would be equally fast and would cost me less per month. Since I installed the USB device, my connection has been less than .10 MB/sec, which is too slow to listen to on-line radio or watch video clips. It took me over 20 minutes to download a song from itunes. The Sprint representatives say the towers in my area are working fine.Would a computer such as mine be able to work properly with such a card? What other factors might prevent it from working properly?