XP does not see WLAN

I have a Zyxel Prestige 650 HWI router with a WLAN card. Accessing the
Internet through the router from my notebooks (running Win98 and
Win2k) works fine.

Now, I try to install a PCI WLAN card on another computer. I have
tried several models and was never able to set up the connection
because XP does not see my WLAN in the Network properties.

Any hints?

Thanks a lot

G.