I updated my Windows Server 2003 running on a VMware image with the lasted April 2009 updates from Microsoft this morning. The result was that none of my SharePoint sites worked afterwards. The solution for me and one of my colleagues turns out to be this old known issue
I would only recommend to disable the Loopback checking for developing environments.
Disable Loopback Checking
- open regedit
- Find HKLM\System\CurrentControlSet\Control\Lsa
- Create a new DWORD value called DisableLoopbackCheck and give it a value of 1
- Restart the computer