Windows Vista Service Pack 1 RC is available through Windows Update. Experience the WU based Vista SP1 installation by following 4 simple steps.
Supported Operating Systems: Windows Vista; Windows Vista Business 64-bit edition; Windows Vista Enterprise 64-bit edition; Windows Vista Home Basic 64-bit edition; Windows Vista Home Premium 64-bit edition; Windows Vista Ultimate 64-bit edition
Also available: The standalone installer. This is available in five-language (English, German, Japanese, French, Spanish) and 36-language versions, with separate downloads for 32-bit (x86) and 64-bit (x64) versions of Vista:
Standalone 5-language, 32-bit
Standalone 5-language, 64-bit
Standalone 36-language, 32-bit
Standalone 36-language, 64-bit
Need more info? Windows Vista SP1 RC release notes | Deployment Guide
Have fun! :)