After some poking around I found that the Wireless chipset in the MacBooks is identified by Windows XP as "Atheros AR5006X". The manufacturer of this chipset has released driver version 4.2.2.4 which supports LEAP, PEAP and various other flavours of wireless security not supported by the wireless driver provided by Apple as part of BootCamp. You can download this updated driver from http://tw2005.giga-byte.com/Communication/Support/Driver/Driver_Wireless_GN-WI01HT.htm
I installed this driver and the included "Wireless Utility" over the top of the Apple wireless drivers for Windows. So far the updated drivers work flawlessly.