Download the most recent Windows drivers from the Netgear site (though recently I could only get the 1.0 drivers to work, not the 2.0 version).
Unpack the downloaded file and cd to the Driver/WinXP directory.
Install the driver with sudo ndiswrapper -i wg311v2.inf
(Check the installation with ndiswrapper -l)
Activate the card with sudo modprobe ndiswrapper
(Check for errors with dmesg)
(Check for available devices with iwconfig)
Open System Configuration>Networking
Press Add and step through the wizard, selecting 'wireless' connection, then 'wlan0' in the wireless device drop-down menu. Enter the Network name and WEP key, which can both be found using Airport Admin in OS X - press the Password button in the toolbar to show the long hex equivalent password, which is what you have to enter here.
Select DHCP in the next step, then finish the wizard - the wireless connection should now be active.
Run sudo ndiswrapper -m so that it will still work after a reboot.