It can be driver related, it may also be hardware related.
If the issue is driver related, go to the manufacturers site and download the latest drivers available for your card.
If hardware is to blame, the card is required to conform to the standard they don't necessarily have to have peak performance. Reading carefully, every card will advertise "Up To 54Mb/s" for 802.11g. The card may just not be up to the task. I don't know the brand myself, but I have seen plenty of disappointing network cards in the past.
If possible, test another wireless card in the laptop and see if you get better performance. You may want to invest in another card if so. (They are relatively inexpensive)
Finally, depending on your testing methods, this issue may actually be between your router and the ISP. Note that you only want to be testing the connection between your laptop and the router. Everything else is out of your control. Also, since WiFi is all about contention, depending on the number of devices currently connected can effect your test. Check your router's DHCP lease table for a list of devices connected.