I just had this problem recently, and did pretty much the same stuff as you: uninstalled and reinstalled (successfully) a bunch of times, and even went so far as to un&reinstall Quicktime itself. But what ended up fixing it for me was deleting the My Documents\My Music\iTunes folder, where iTunes Music Library.xml and iTunes Library.itl are stored. I played around a bit, and determined that those files were what was causing iTunes to crash on startup every time. Guess they get corrupted once in a while. Hope that works for ya.
