I have found that the only way I can get my Windows Mobile 5 device to connect to ActiveSync 4.5 running on Windows XP SP3 is to uncheck the checkbox entitled "Enable advanced Network functionality" on the Connections/USB to PC tab of the Settings app in Windows Mobile 5.
This failure occurs even when the Onecare firewall is turned off.
The advanced Network Functionality worked prior to installation of Onecare.
I should add that while attempting to isolate the problems I have been experiencing with network browsing, I disabled the connection that had been in place for the usb interface to the windows mobile device. However, at some point after being disabled, this connection disappeared from the connections list.
while using the network connection wizard to try to figure out how to get the USB connection reinstantiated, the previous, disabled instantiation of the USB connection suddenly reappeared (I kid you not!). I was thus able to re-enable it, and thereby get the Windows mobile "advanced network functionality" to work again.
I suspect that the reason that the USB connection disappeared after disabling it is that, by disconnecting the Windows Mobile device from its USB port, the OS concluded that the hardware was no longer present. Why it did not immediately reappear after reconnecting the USB cable is an unsolved mystery that I will let lie for the time being.