I had this problem and I tried everything. I went without itunes for about 2 months. I finally figured out that Quicktime controls alot more of your computers settings than I realized. First go into the settings of Quicktime and see if there is something wrong. If that doesn't work, then do what I did.
1. Uninstall everything that has to do with itunes and quicktime (and apple, if possible). Most importantlty, clear your registry.
2. Download Quicktime Alternative or Media Player Classic.
3. Download itunes installer and save to desktop. Don't run installer yet.
4. Clean hard drive. CCleaner is one of the best free programs EVER.
5. Unplug from internet. This will make it safe to shut down your anti-virus and anti-spyware that runs in the background. Don't forget to turn back on when going back on-line
6. Install QT Alternative, then install itunes. This woked for me. All the programs above can be found at download.com. Don't forget to set a System Restore Point first.