Updated iTunes, now it won't start at all

I had last used iTunes about a week ago. Last night, I went to open it again, which it did fine, but then immediately informed me there was a new version (which it seems to do almost weekly!) I told it to get the update, it took me to the site, I downloaded the Windows version, and let it start the install. Somewhere near the end, while installing (yet another) copy of QuickTime, it stalled and wouldn't finish. I had to force the PC to shut down. When it rebooted, I tried to start iTunes, got the hour glass cursor, and then the machine locked up solid...no keyboard or mouse input accepted. Had to force another shutdown. When I got it back up, I decided to retry the installation. This time it got to the final screen saying the install was done, but at that point, machine was again locked from accepting keyboard or mouse input. Powered down again, rebooted, and tried to start iTunes. Again, hour glass cursor appeared, then system locked solid. It was working fine before the install (note that iTunes originally opened up fine, before telling me there was a new version), but now I can't get iTunes to start at all, and reinstalling didn't seem to help. I don't see any way to get "official" Apple help on this site, as it seems to be limited to this forum. So what is the next step in correcting this mess?? Note that the machine always has Norton AV and Windows Defender running for protection, and is scanned pretty regularly with Windows Defender, Spybot S&D, and Pest Patrol, so doubtful there's anything bad on the system. I did disable Norton AV during the second install, in case that was a problem, though I've never needed to do that before. But right now, system refuses to start iTunes successfully.