iTunes won't install?

Today I went to open my iTunes and got an error (something about the Library being for another version or something, I forget, I already uninstalled iTunes), so I uninstalled it (which was a major hassle) and went to reinstall it. Anytime I try, the Installer either crashes or I get a message that says

"The Installer is damaged.

The Installer can’t open the package. There may be a problem with file ownership or permissions."

What is this? I've rebooted my computer multiple times and I still get this error. I've downloaded the installer three times, and all three give me this message. I even went back and tried to use the installer I used months ago to install iTunes 10, and even that gives me the message. Help!!! D:

I run SL 10.6.4, and also run Win7 through Parallels 6. I thought maybe since I'd installed iTunes on the virtual machine maybe that's why it was giving me problems, but I uninstalled that, rebooted, tried to install on my mac, still to no avail

Any help would be GREATLY appreciated!!!

I would say download and install the 10.6.5 combo update, but that probably won't work either.

I'd try booting to your OS X disc and run Repair Permissions from there.

Then try booting into Safe Mode.

If those don't work, I'd probably restore from my backup.
Don't have a backup, probably easier to do an Archive/Install than to continue troubleshooting. Make sure you create a backup first, preferably bootable and make sure it works.

That did the trick! Thank you sooo much! I was panicking, as I've already gone through two harddrives... ugh.

What causes this to happen, does anyone know? So I can avoid it in the future... haha

Which step worked?