slow wireless network sharing
 

I have a wireless network between my desktop computer with windows vista and my laptop with windows xp.
the vista computer is brand new , and I have installed the Xp computer yesterday.
my router is motorola sbg900.
The internet is working great in the laptop.
the file sharing is the problem, it takes about 10 minutes to transfer a 50Mb file between the computers.
I've turned off the firewalls in both computers and in the router, I've tried to switch channels but nothing helped. the signal is excellent.
can anyone help?
Thanks