Slow file sharing when using 802.11g

I formed a home network using and Mn-700, MN-720 and Mn-
730 (3 computers, 1 wired and 2 wireless). Both computers
are using WinXp Pro and have all of the security updates
from microsoft. Both can connect to the internet and see
others in file sharing. Internet speed is ok but when I
trying to open the share folder in the other computer, it
takes abour 10 seconds to display a folder or select a
file from the network computer. The firewall in winxp and
all antivirus programs have been disable and but the speed
is still slow. I don't have any other firewall installed
except the Mn-700 router built-in firewall. Anyone can
help?