Hi B Noir,
Unfortunately I tried both of those methods already a while ago and neither worked. I suspected it was some corruption in my registry, as I noticed a few other oddities within my Windows system (such as seeing folders or files deleted in Windows explorer not appearing to be deleted until after I manually refreshed the window). Since I couldn't isolate the particular registry problem, the only thing I had left to do was to reformat my OS drive. I did that, and it seems to have fixed the problem, as I have now installed quicktime without issue.
Sorry for anyone else looking for a simpler, less destructive solution, but reformatting really isn't all that bad as long as you properly back up what you need...