well fellow luke check the download from itunes.. watch it download and see if it stops midway.
What your saying sounds like a file thats incomplete or messed up..
try downloading other programs?
Try downloading to another drive (could be a drive going bad?)(unlikely but to check)
Whenever I try to install any iTunes (besides 6.0 but it's to outdated for me to put music on my iPod) it comes up with a box that says
"This installation package could not be opened. Verify that the package exists and that you can access it, or contact the application vendor to verify that this is a valid Window's Installer package."
I've tried everything and I can not get it to work.