I had been trying to install Office 2003 SP3 into my new Thinkpad T61 laptop but kept failing. When trying intallation from Windows Update, it started normally but prompted error on the half way, and never gave a detailed explanation.
I thought it might be Windows Update's fault and so download the separate installer from MS site, but the error persisted, except it gave a detailed error message somethign like 'no sufficient permission to c:\windows\system32\mapisvc.inf'.
I checked the ACL of above mentioned file and found its owner is 'TrustedIntaller', which actually stand for Windows installer service instead of a real group in VISTA. And this file was read-only to the local administrators.
So I made it owned by administrator, and grant write to administrators group. The the SP3 insallation was flying!
Here is the detailed explanation of 'TrustedInstaller'.