Microsoft Knowledge Base Article 832161 contains the following summary:
This article describes a problem where you experience long delays when you log on to the domain and when you open network shares. There are a variety of possible causes. These causes include problems with the WebClient service, problems with proxy settings, and problems with the server configuration.
One easy test that you can use is to turn off the client computer's WebClient service or the server's service that is listening to Transmission Control Protocol (TCP) port 80. If either of these workarounds resolves the problem, there are ways that you can re-enable these services and reduce the delay. Update to the latest service pack for Windows XP, bypass the proxy server for local servers, change the order of network providers so WebClient is the last provider, install any updates to the WebClient service, and then make adjustments to your Internet Explorer configuration.