Right, I've just got around to reprovisioning the XP pool. Once the machine had stopped on the desktop, I did an ipconfig all and the media state is showing as "Disconnected".
Looking at the PortGroup in vSphere I can see the VM listed with a valid port ID, but the "State" is shown as "--", so the machine isn't on the network. I've double checked and there are plenty of IP's available.
For info, the Network Adapter is a "VMware Accelerated AMD PCNet Adapter".
Any ideas would be greatly appreciated. 