A reliability and performance update is available for Windows Vista SP1-based computers

Update information
An update is available that will help improve the performance and reliability of Windows Vista. If you are not sure whether Windows Vista Service Pack 1 (SP1) is installed, see the "How do I find out whether I have Windows Vista SP1 installed?" section.

To install the correct version of this update, you must know whether your Windows Vista platform type is 32-bit or 64-bit. If you are not sure which platform type is installed, see the "How do I find out whether I'm running a 32-bit or 64-bit version of Windows Vista?"
http://support.microsoft.com/?kbid=952709