You should try downloading the installer again.

If this does not do the trick, are you logging into Vista under a limited account with no Administration privileges? If so you should have the User Account Control turned on:

< THIS IS A WINDOWS VISTA ONLY PROCEDURE - Taken from Vista Help >

 

User Account Control (UAC) can help you prevent unauthorized changes to your computer. It works by prompting you for permission when a task requires administrative rights, such as installing software or changing settings that affect other users.

 

We don't recommend turning User Account Control off. If you do turn it off, you should turn it back on as soon as possible.

 

1. Open the User Accounts item in Control Panel
2. Click Turn User Account Control on. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. Select the Use User Account Control (UAC) to help protect your computer check box to turn on UAC, or clear the check box to turn off UAC, and then click OK.

Let us know if this helps.