OK, go to this article and start at the section titled
*General installation troubleshooting*
and go through the steps in order. You have to click on each section for it to expland. If one step gives errors, just go onto the next step.
Also I'd turn off any AV software (like Norton, MsAfee, NOD32, Kapersky) before starting this.
And for the love of god, back up your itunes ITL file....put a copy on your desktop or a USB drive.
http://support.apple.com/kb/HT1926