You can't install Vista SP1 via Windows Update?

When you try to install Windows Vista SP1 via Windows Update you can get error: Windows Vista Service Pack 1 is not available for installation from Windows Update and is not offered by Automatic Updates.

To help ensure a positive update experience, Windows Update will temporarily not offer Windows Vista SP1 to systems that have the following device drivers. In some cases, these device drivers are problematic on Windows Vista-based computers when you update to Windows Vista SP1. By installing updated device drivers, you will resolve the issue and enable Windows Update to offer Windows Vista SP1.

Here are the drivers that doesn't work well with Vista SP1:

Audio drivers

Realtek AC'97

• For x86-based computers: Alcxwdm.sys - version 6.0.1.6242 or earlier
• For x64-based computers: Alcwdm64.sys - version 6.0.1.6242 or earlier

SigmaTel

• For x86-based computers: Sthda.sys - version 5.10.5762.0 or earlier
• For x64-based computers: Sthda64.sys - version 5.10.5762.0 or earlier

SigmaTel

• For x86-based computers: Stwrt.sys - version 6.10.5511.0 or earlier
• For x64-based computers: Stwrt64.sys - version 6.10.5511.0 or earlier

Creative Audigy

• For x86-based and x64-based computers: Ctaud2k.sys - version 6.0.1.1242 or earlier
• For x86-based computers: P17.sys – all versions (This was originally a Windows XP-based driver.)

Conexant HD Audio

• For x86-based computers: Chdart.sys - version 4.32.0.0 or earlier
• For x64-based computers: Chdart64.sys - version 4.32.0.0 or earlier

Biometric (Fingerprint) Sensors

• AuthenTec Fingerprint Sensor with the Atswpdrv.sys driver file – version 7.7.1.7 or earlier
• UPEK Fingerprint Sensor with the Tcusb.sys driver file – version 1.9.2.99 or earlier

Display drivers

Intel Display

• For x86-based computers: Igdkmd32.sys – versions between and including driver 7.14.10.1322 and 7.14.10.1403
• For x64-based computers: Igdkmd64.sys – versions between and including driver 7.14.10.1322 and 7.14.10.1403

Other drivers

Texas Instruments Smart Card Controller with the GTIPCI21.sys driver file – version 1.0.1.19 or earlier

Sierra Wireless AirCard 580 with the Watcher.exe application – version 3.4.0.9 or earlier (This application is located in the AirCard 580 Program Files folder.)

Symantec software driver for Symantec Endpoint Protection and for Symantec Network Access Control clients

• For x86-based computers: Wgx.sys – versions 11.0.1000.1091 or earlier
• For x64-based computers: Wgx64.sys – versions 11.0.1000.1091 or earlier

Note Symantec is aware of this issue, and it is working on a solution. Symantec provides various update procedures. This includes their LiveUpdate service.

Edit:

TheGreenMan has posted some more reasons on another thread.

Quote

You are already running Windows Vista SP1.
Windows Vista SP1 has not been released for the language of the language pack that you have installed.
Windows Service Pack Blocker Tool is used to block the delivery of Windows Vista SP1 from Automatic Updates or from Windows Update.
You tried to install Windows Vista SP1, and the installation failed with a known inconsistency in the file or registry structure.
A hardware device driver or device software was problematic when you updated to Windows Vista SP1.
The Windows Update service can detect the small set of device drivers and software that falls into this category. The Windows Update service will not offer Windows Vista SP1 until an update for the hardware device driver or the device software has been installed.
You have installed a prerelease version of Windows Vista SP1, and you must uninstall the prerelease version, or start with a new installation of Windows Vista.
You used the third-party program vLite to configure the system, and you may have removed required system components that have to be available for Windows Vista SP1 to be installed.