Have you tried using Run as adminstrator when you run the installer? (Right click on the installer to select this).
Otherwise do you have another adminstrator account you could try installing from? SOmetimes using another account works especially if iTunes was originally installed using another account.
If none of that helps, take a look at this post on using the SubInACL tool to repair registry permissions:
http://blogs.msdn.com/astebner/archive/2006/09/04/739820.aspx
Note that Vista is not in the list of supported operating systems for the SubInACL tool. If you read some of the blog posts, it appears that some people have used it with Vista, also if you Google - SubInACL Vista you get some hits to look at.
I have not used the SubInACL tool myself and I do not know how safe this procedure is, although some people on this forum have used it successfully.