Even today, iTunes still depends on QuickTime, but given I haven't used Windows in almost ten years or seriously used iTunes in almost 8 years, I couldn't tell you whether that dependency is the old QuickTime 7 API or if Apple ported a subset of the newer QuickTime APIs over I also couldn't really tell you which they use on Macs* anymore either given I prefer to avoid launching iTunes whenever possible.
That said, iTunes up to about version 6 point whatever the last point update was, was genuinely a really great jukebox on Windows. It was easy to use, loaded fast, ran fast and I ran it nearly 24/7 on my old XP machine. iTunes 7 ruined all of that, and my first indication would have been when it took over an hour to complete the install process and the first run was dog slow.
* Apple did deprecate and remove QuickTime 7 from Mac OS X, that said, I wouldn't put it past them to maintain an API in maintenance mode exclusively for their own use, particularly when it comes to maintaining the many-heads of the beast that is iTunes.