There have been a number of reports of networking issues after installing SP2 for Windows Server 2003. We now have a fix that will address these. The errors you may see include:
Error 800: Unable to establish connection.
These are all resolved by this fix. From that page, as to what causes these problems.
These issues occur because of several problems with the Windows Server 2003 SNP features that are enabled in Windows Server 2003 Service Pack 2. These features include Receive Side Scaling (RSS) and TCP/IP Offloading. Specifically, these problems include the following: