The latest update to iTunes fubar'd my computer recently. I tried to use the automatic updater when it recommended I update to the newest version and partway through the update everything went to hell. I got missing .dll errors, iTunes kept crashing, and I would get authorization errors during the install process.
I went through a system restore, which didn't work. I reinstalled iTunes from a freshly downloaded .exe and was still having problems. Just as a warning to anyone else who runs in to this, what it took was completely uninstalling and reinstalling iTunes. Uninstalling was more complicated than I had thought it would be.
If anyone else runs in to this issue, follow the instructions here:
http://support.apple.com/kb/ht1925
Uninstall the apps in the order that they recommend and then start with a fresh .exe from the iTunes website. After doing all that I seem to be in the clear. Fortunately all my settings, play counts, apps, etc. seem to have been preserved in the process and my iPhone still syncs ok. It was a major hassle that took several hours to figure out. A coworker ran in to the same issue so I wonder if it's something with the most recent update.