“Today, we’re making it available to anyone interested in testing SP2 for Windows Vista and Windows Server 2008 prior to final release. 
You can download the Windows Vista and Windows Server 2008 Service Pack 2 RC from our Customer Preview Program site or install through Windows Update using our Windows Update Experience Kit,” writes Microsoft’s Brandon LeBlanc.
The RC SP2 has been available since last week, but Microsoft made sure at the time that only TechNet and MSDN subscribers were allowed to download and install it.
Be warned though: if you have the SP2 for Windows Vista and Windows Server 2008 Beta installed, you will need to uninstall that first before installing the RC.