Vista - problem installing itunes - can't create "installer cache" directo

Itunes used to work on my computer. It stopped working and I uninstalled it
but am having trouble reinstalling it.

I keep getting a message which reads "an error occured while attempting to
create the directory c:\ programdata\apple\installer cache".

I've tried saving the setup file on my desktop and running it as
administrator but the same message appeared. I've tried downloading the
latest updates of vista but this made no difference. I've also tried
disabling some items using the msconfig, which hasn't worked either.

Does anybody know what this means or how I can solve this problem?