Hi @mcaspar . Changing a computer from ADSL to NBN should need no changes to Windows.
Are you connecting via ethernet or wifi?
On Win XP, can you check the network settings. I can't tell you the exact procedure for XP but is something like this.
Control Panel, Network, right click the ethernet adapter and click Properties. There will be several items in the list: Client for Microsoft Networks, etc. All should be ticked except maybe Internet Protocol Version 6. (The wifi adapter might have IPv6 ticked by default.)
Click Internet Protocol Version 4, then Properties. Select "Obtain an IP address automatically" and "Obtain DNS server address automatically".