Unable to install Service Pack 1 on Windows Vista using Windows Update

If you can read Spanish, you may like to see Windows Vista no es capaz de instalar el Service Pack 1 mediante Windows Update.

Some Windows Vista users reported difficulties to apply Service Pack 1 using Windows Update. They installed this Microsoft operating system from scratch using installation media with no integrated service packs (RTM version). After applying several updates, Service Pack 1 is offered with a size of only 1.9 MB, much smaller than usual. When this service pack is installed from Windows Update, the setup wizard starts, but the installation process finishes too quickly and it does not demand a restart. Consequently, Windows version information reports no Service Pack 1 installed. Therefore, Windows Update offers Windows Vista Service Pack 1 repeatedly.

The version number of Windows Vista with no service packs is 6.0.6000, where 6000 is known as the build number. Windows Vista with Service Pack 1 is version 6.0.6001 and Windows Vista with Service Pack 2 is version 6.0.6002. Several tools can be used to get the Windows version number and the current service pack level:

System information tool, Msinfo32.exe.
Windows command prompt, Cmd.exe, which shows the Windows version number in the header. The VER command can also be used.
Windows version information, Winver.exe.
Control Panel, System and Maintenance, System. You can also open System by pressing Windows+Pause in your keyboard or right-clicking Computer and then clicking Properties.

The reason of this Windows Update behavior is unknown to me. However, the fix is very simple: the full Service Pack 1 setup should be downloaded. There are four service pack variants: 32-bit with five languages, 64-bit with five languages, 32-bit with all languages and 64-bit with all languages. The packages with five languages include English, French, German, Spanish and Japanese only, so they are smaller than their full language equivalents. A full language package is required when a language pack other than English, French, German, Spanish or Japanese, is installed on the computer. You must also choose the SP1 package which matches the «bitness» of your Windows Vista-based system: Is my PC running the 32-bit or 64-bit version of Windows?

Service Pack 1 is mandatory for Service Pack 2. Because Microsoft no longer supports Windows Vista RTM and Windows Vista Service Pack 1, Service Pack 2 is required to fully update a Windows Vista based system. Furthermore, many recent applications request at least Windows Vista with Service Pack 2. Right after you apply Service Pack 1, Windows Update will install some more updates and then will offer Service Pack 2. Unless there are any uncovered issues, Service Pack 2 installation should complete smoothly. I include the Windows Vista Service Pack 2 download links for your convenience. These links are also valid for Windows Server 2008 but not for Windows Server 2008 R2:

Note that applying a Windows Vista service pack does not remove old files from an earlier service pack nor the RTM version of the operating system. Those old files remain at the component store in case you want to uninstall the service pack. You can use a special tool to reclaim disk space. The tool must be run on an elevated command prompt window as an administrator. If you have just installed Service Pack 1, you must use Vsp1cln.exe. Service Pack 2 carries Compcln.exe, which is an enhanced version of Vsp1cln.exe.

If you have applied Service Pack 1 and you plan to upgrade to Service Pack 2 right away, you don’t need to delete the Windows Vista RTM files using Vsp1cln.exe first. Nevertheless, this can be useful when disk space is tight and you want to make some room before you upgrade to Service Pack 2. Compcln.exe can remove files from both RTM and Service Pack 1. Once the component cleaning process is started, it must not be interrupted, it cannot be undone and the service pack will be made permanent. You cannot uninstall the service pack thereafter.