If you save the iTunes setup program to your drive and then try to install by
opening that file, you will sometimes get the message "not a valid Win32
application". Instead of saving the download, select "run" and the program
should install normally. If it still won't install, see below.
I had the same problem with iTunes/QuickTime and Roxio/Sonic. Before Vista
will let you update/install a newer version of a program, all remnants of the
program must be completely removed from your system. Since the Vista
uninstall feature sometimes does a poor job at this, these remnants often
have to be removed manually. The following tutorial shows how to do this:
http://www.vistax64.com/tutorials/87249-unable-install-latest-version-program-vista.html