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.

Two relatively inexpensive WPA-PSK clients that come to mind for Win2k is the Odyssey client from Funk Software (~$50) and Aegis client from Meetinghouse (~$40)

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: »···8-me.exe
Windows 2000/XP: »···0-xp.exe