Vista PC keeps disappearing off network

I have just built a desktop PC with Vista Ultimate, and good spec hardware. I
also have a laptop with XP Home. They are both networked wirelessly via an
Edimax ADSL modem/router. The networking works well - both PCs have a good
signal, both can reach the Internet, and the Workgroup is set up OK as I can
share resources between the PCs.

I occasionally need to use Remote Desktop to access the Desktop PC from my
laptop. However, I noticed that I got very frequent slow response and also
network loss being notified in the RD client. To investigate this, I used a
Cmd prompt on the XP laptop to run "ping -t <desktop>" on the laptop. This
showed mostly good ping times (<10 mSec), but frequently a ping would take a
long time (>1000 mS); and I would also frequently get ping timeouts on a
number of successive pings.

I tried the same in the other direction, on the Vista desktop, by running
"ping -t <laptop>", and got consistently good respopnse times with NO packet
loss.

As a further test on the laptop, I ran a "ping -t <router>". This gave
consistently good times and no packet loss.

So, it seems there is an intermittent (though very frequent) problem
confined to connectivity between the laptop and desktop, characterised by
poor ping results in one direction only.

Can anyone suggest a possible cause for this?