Windows 2000 does not support WPA. However some wireless cards (Netgear 511T) actually ship with support for WPA in Win2k. There also appear to be several 3rd party addons:
Wireless Security Corporation (which, it seems, has recently been purchased by McAfee) provides a free WPA supplicant.
You can also download the free supplicant for T-Mobile Hotspots. It also supports non-T-Mobile connections and provides for WPA and 802.1X.
Windows 98/Me: » client.hotspot.t-mobile.com/TMCl···8-me.exe
Windows 2000/XP: » client.hotspot.t-mobile.com/TMCl···0-xp.exe