At any point when there is a new version of iTunes for upgrade, I upgrade. When iTunes starts after the upgrade, it doesn't see other computers on my network, or our AppleTVs. Bonjour (mDNSResponder.exe) is running, but not working. I either 1) manually shut it down in my Task Manager & try to restart it by double clicking on the *.exe, or 2) stop & restart it in Services.msc. Bonjour still doesn't work in that AppleTVs & computers on our network cannot see each other. No sharing is happening. We stream a lot in our house, and when AppleTV isn't seeing a PC or Mac online, or vice versa, it's a problem (of course).
FYI: Just a few moments ago, I reran the latest iTunes installation file (downloading the exe straight from the Apple site), choosing the "Repair" option. Nothing changed. I should not have to uninstall & reinstall iTunes every time just to get Bonjour to kick in again. It seems that Bonjour never liked an iTunes upgrade, as it stops working & sharing when an upgrade happens.
I'm on Windows XP Pro SP3 with all the latest updates, etc.