I had a ton of troubles using windows update on the 32 bit version of Vista Home Premium, and came to this solution.
1) restart your computer
2) go to C:/Windows/SoftwareDistribution/Download and delete all the contents of this folder
3) go to C:/Windows/SoftwareDistribution/DataStore and delete all the contents of this folder
4) run Windows Update
5) view available updates
6) right click on each update, and go to the information url
7) one by one, manually download and install each update
8) if the url is invalid, which happens frequently, try that one another day, until it does work. can sometimes take a few days
I know, a pain in the butt, but that was the only way I was able to assure I never had the problems with Windows Update. Since switching to the x64 Vista, I have not had any problems with Windows Update though.