Windows OS

If you've downloaded KB 941649 previously and did not experience any issues, you don't need v2.
If you had Serial ATA/AHCI issues, typically in combination with optical drives, with the original KB 941649, grab v2 and give it a try.

Technical differences between the two
This package has been updated to version 2. An issue was identified in which certain computers that have specific combinations of older AHCI USB chipsets and CD or DVD drives that use the SerialATA interconnection bus may not start after the installation of the original version of this package. Version 2 of this package has removed the fix that caused this problem. If you are using a computer that experienced this issue, you can restart the computer by using a bootable Windows Vista DVD and by selecting the Repair/System Recovery options. If your computer has already installed the original version of this update and has successfully restarted, you do not have to install version 2 of this update.

By the way, unrelated to this, Service Pack 1 for Vista will include all of these fixes, and boot time, at least for my test boxes with early versions of SP1, is dramatically sped up. Ditto for the duration between entering your user credentials and getting a usable desktop.