Slow Wireless connection - HELP

I recently setup my son's PC (running XP) with a wireless card (Dlink PCI)
that works with a DLink DI514 wireless router. I used the DLink utility
instead XP to configure it and have turned off WEP encryption.

The icon typically shows a connection at 11Mbps. Despite the fact that the
signal appears to be fine (60-70% strength and 75% quality) the internet
access seems to run MUCH slower than on other hardwired or wireless PCs in
the house. The internet access is via cable modem (combo w/VoIP although
little use) from Optimum on-line. I usually see over 3-5Mbps on bandwidth
tests on my hardwired PC as the subscriber density in my area is still very
low.

Any suggestions or thoughts on whats causing the problem and how to remedy
it.