theres a Hp Pavilion 5500 desktop and laptop so depends which you have--for desktop a pci usb card will give up to about 6 usb ports and for a laptop if it has a pcmcia slot a pcmcia usb card will give up to 4 usb ports
Just wondering, are ALL the ports not working? You might check your ports and see if any one in particular is broken. Sometimes when the center divider plastic is broken, the pins will short out against the shield, and cause other ports to quit working. If one port in particular is the problem, you can bend the pins away from the ground shield, and the rest will start working. Just be sure not to use that broken port again if that's the case.
Mav is right sometimes this is the case also sometimes the ports do not output enough voltage to one particular port, just be cautious of that when you use up all your usb ports, also if it is a newer version laptop you will be looking at a PCMCIA-Express to usb card and not the older standard PCMCIA connection, just make sure.