i am having this same problem. "itunes store stopped working" and closes itunes after upgrade to 8.1. I uninstalled all my apple sw (in the presribed order) and reinstalled and still am having the same problem.
i am going to try the manual fix suggested above (copying QTMovieWinn.dll), and will report back, but I wonder why this should be the source of the problem. Has anyone at Apple recognized this problem???
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OK the fix seems to work so far, but beware those of you who installed itunes to a custom folder during installation, copying the file into C\:program files\itunes does NOT do the trick, you have to (also??) put it in the custom folder you installed to. Might seem obvious, but thought I'd share nonetheless.