Have Windows XP. Installd 7.6 and had many similar problems - couldn't import music, couldn't access the internet, outrageous CPU usage, etc.
After reading through all the suggestions listed, this is what I did.
1. renamed the itunes library.itl file. Find it in \Documents and Settings\username\my Documents\My Music\Itunes. I just called it old itunes library.itl.
2. Totally uninstalled Quicktime
3. Downloaded the installer for Itunes 7.6 and Quicktime to my desktop from apple.com
4. Reinstalled Itunes and Quicktime
5. Restarted computer, but had to force a shutdown.
6. Went back to the itunes folder and renamed the newly created itl file.
7. Renamed the old file back to it's regular name.
8. Restarted itunes. Everything is working again.
I do have to say that I only have music files (about 6000) and I've noticed that itunes has slowed down a little, but no more hangups, I can access the internet, import songs, and do everything I used to do, just a little slower. Don't know if all these steps are necessary, but it worked for me. I have shut down normally and restarted my PC a couple of times and it's still working. My thanks to everyone on this site.
It has been an aggravating evening. Hope this helps.