Weird problem with wifi, PC freezes

Hi all,

I have a Thomson SpeedTouch 585i ADSL wireless router at home. I
installed a WiFi PCI card to my desktop PC and works fine. I can
access the Internet and all.

The weird thing: when I switch on my laptop, it gets immediately
connected to the Internet BUT my desktop PC, if it is switched on and
connected, looses its connection to the Internet and keeps freezing
(!) instantaneously every 2-3 seconds...

In other words, both the laptop and the desktop work fine if working
separately. They cannot work simultaneously though. I brought a
friends laptop at home and the 2 laptops work together like a charm.

Is it a problem of the desktop's PCI WiFi card? It does work though...
(if on its own...).

I'm completely confused...

Any help would be greatly appreciated. Thank you!!!

Nicolas

P.S. My router uses DHCP and both devices are instructed to Obtain IP
automatically.