Are you running it as an admin? I never liked QT or Itunes. I hated it so much, I bought my wife her own laptop to keep that crap on.
Try disabling your AV first. If that doesn't work, boot into Safe Mode w/ networking to install (tap F8 when your computer boots - once every second).
Apple software sucks, I have an iPod so I'm stuck with iTunes. For no more than what it does it's slow and a resource hog. I've caught it using over 2GB!!! Here's some steps I found that might help you out: http://support.apple.com/kb/HT1926