When you upgrade a guest operating system that is running under Microsoft Virtual Server 2005 to Microsoft Windows Server 2003 with Service Pack 1 (SP1), you may experience decreased performance in comparison with the original-release version of Windows Server 2003 as described here.This issue occurs because the Virtual Machine Additions included with Virtual Server 2005 does not have the performance optimizations that are required for Windows Server 2003 with SP1. Virtual Server 2005 was released many months before the release of Windows Server 2003 SP1.

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VMAdditionsENT.msp
VMAdditionsSTD.msp