Microsoft KB article 943899 describes the fixes that are available in this update. Here is how Microsoft describes it:
This update improves performance, responsiveness, and reliability of Windows Vista in various scenarios. This update resolves the following issues on a Windows Vista-based computer:
• You receive a "Stop 0x000000A0" error when you try to switch the computer to the hibernate state.
• You receive a "Stop 0x0000009f" error when you switch the computer to the hibernate state or to the standby state. Or, you receive this Stop error when you resume the computer from the hibernate state or from the standby state. This problem occurs on a computer that has a wireless network connection.
• The disk does not spin down after a specified time of inactivity.
Additionally, this update can help improve performance when you perform operations that are related to large disk I/O. After you apply this update, you may notice up to a 15 percent performance improvement in some copying operations and when moving some large files.