On 64 bit versions of Windows 2003 and XP Professional you can configure IIS to operate in either 32 bit or 64 bit mode. If you install the 64 bit version of IISxpress when IIS is configured for 32 bit mode you will see a "Server Unavailable" message appear when any requests are made to your web server.
This problem is caused because IIS cannot load the 64 bit IISxpress ISAPI DLL when in 32 bit mode, this effectively stops IIS from working. To recover your web server you can either uninstall IISxpress or switch IIS into 64 bit mode.
Currently the IISxpress 2.0 32 bit installer will not install on 64 bit operating systems, we plan to release a new version (2.0.1) shortly which will install on these systems. We expect this release will correct the problem.
For more information on this issue please see: http://support.microsoft.com/?id=894435, this article explains how this issue also affects 32 bit and 64 bit installations of the .NET Framework.