Before and after updating Apple s/w, including the OS, repair permissions in Disk Utility or Onyx. Aways!
Sometimes, the s/w updates thru Apple's s/w update app are...well, broken. Wait a couple of days before updating and read the comments on MacIntouch.com which you should read every afternoon or evening, anyway. (This advise is probably OBE'd in your case.)
Try downloading the latest iTune update from Apple's download site and try again. I think each iTunes updates are the full current version of iTunes. (Please compare file size as I could be wrong. I was once, along time ago.)
If none of the above works,
- Delete you /Extra folder;
- Re-install 10.6.6 with the 10.6.6 Combo updater BUT, don't let it re-boot;
- Re-run MultiBeast and re-boot;
- Repair permissions; and finally..
- Re-install iTunes with the download from Apple's iTunes (dedicated) page, not an update (and, optionally, repair permissions again).
Hope this helps. It's a pain when a situation like yours happens. But, be advised, it also happens to Mac owners as well as with other operating systems and apps in those OSs. I've never had this happen, but I've read the war stories.