I had a similar problem trying to download the update for Norton Security. I was told by them that by default Vista does not create an "administrator" account. So even though my account was listed as "administrator" (in the User Accounts) it is not. I was told that you have to create an admistrator account in order to get the updates. I can tell you how to do this, but the information is on my other computer. It involves using the "command prompt" to create the "administrator account", It's quite easy.
Here's how to create an Administrator account. 1) Type "command" (no commas) in the Search bar. 2) Right click "Command Prompt" and select Run as Administrator. 3) Type the following command "net user administrator /active:yes" (no commas).
4) The command prompt will now display. Click the "x" in the top right corner of the Command Prompt box to close it. 5) Restart the computer and you should have an "Administrator Account". Log into this account for troubleshooting or for anything that requires administrator privileges. According to the article I got this from, you should only use this account when necessary, for security reasons, protection of your computer etc.