First thing, I haven't used iTunes for a very long time, but you shouldn't need to use it.
You have mentioned that you have HTC Sync, which means that you will have installed the HTC drivers on your PC.
The easiest way to transfer anything is to just connect the USB cable to your phone and PC, then go to My Computer and you should see your phone showing. Open the phone folder and either go to the Video folder (Music, Documents, Pictures, etc.) or create a new folder. Then just copy and paste the files from your PC to your phone.
The good thing about this is that you can quite happily create as many folders as you like to organise the files the way you want them.
Like I said at the beginning, I don't use iTunes and when I needed it I usually used SharePod which was less restrictive, so I hope that I haven't misunderstood your question. But if you need any more help feel free to ask.
