Have you checked that any firewall settings are still OK, particularly if you have specified IP addresses which the router's DHCP server may have changed?
To save a lot of hassle, it might help to use "IPX/SPX/NetBIOS Compatible Transport Protocol". This doesn't need configuring with IP addresses and so on like TCP/IP and isn't affected by firewalls. However, for internet access, TCP/IP is still required. To use IPX/SPX for Local Area Network connections, it must be installed in each computer in the network.
To install "IPX/SPX/NetBIOS Compatible Transport Protocol" in Windows XP, go into the Network Connections folder, right click the Local Area Connection and select Properties. In the General tab, click the Install button. In the next box, highlight Protocol and click Add. Select "NWLink IPX/SPX/NetBIOS Compatible Transport Protocol" and click OK.