Download the x86 (32 bit version) , a 434.5 MB download or the x64 (64 bit version) a 726.5 MB download.
Note that the download size from Windows Update of Windows Vista SP1 for x86 and x64 is 65 MB and 125MB respectively so if you have one PC, it should be faster to use Windows Update. See ” Things to know about Microsoft Vista SP1”
Also see the overview of Windows Vista SP1 with details such as release notes, deployment guides, etc.