I vaugley remember having this problem myself.
Is this a fresh download of Itunes or an old one.
If its new, try searching around for an older version.
If its old, download the latest one.
I think what has happened is some how the installer is corrupt and cant be found, it was a long time ago when i had this issue, it was actually when i was using the beta builds of 7 before it was released.
But yeah, try one of the options i just stated above.