If you run Windows Vista and unable to update iTunes 


1. Download subinacl from:

http://drpcdr.ca/subinacl.zip


2.  Extract subinacl.msi file

3.  Install subinacl.msi

4.  Download command script from:

http://drpcdr.ca/resetacl.zip


5.  Extract resetacl.cmd file 

6.  Click on resetacl.cmd

7   Wait 10 minutes for resetacl to complete.

8.  Install iTunes 

Source:

http://blogs.msdn.com/astebner/archive/2006/09/04/739820.aspx