Sync problem over WPA WiFi connection

I have an HP iPAQ HX4700 PPC. It will synchronise OK via the cradle with
ActiveSync 4.2. However, I want to be able to synchronise it via a WiFi
connection.

With a previous (Win 2K) computer and the wireless access point using
WPA-PSK security, the PPC would synchronise OK using ActiveSync 4.2.

However, I now have a new (Win XP SP2) PC, and the same wireless access
point is now using WPA security. The PPC will connect to the access point
and can use it to browse the internet and all PCs in the LAN domain, but the
PPC will not synchronise. When I attempt to synchronise, the PPC does nothing
for about a minute, then displays the following error message:

Communications Error
Cannot start communications with the desktop computer. Reconnect your
device...

I do not have any firewall software installed on the PC beyond the Windows
Firewall included with Windows XP

The PPC is using a static IP address (192.168.0.10), gateway
(192.168.0.254), DNS and WINS server (192.168.0.1), not DHCP. The PC's LAN
IP addresses are 192.168.0.2 and 192.168.0.7 (dual ethernet ports that cannot
be teamed on the existing network).

Can anyone help?