Your download may have been corrupted (crazy as that sounds - you'ld think it just wouldn't work at all then!) and so essential bits are now missing but QT doesn't know they're missing, if you get my drift. Try downloading the installer again or pulling it off a cover disk instead.
I'm not familiar with whether there's an uninstall option on the PC version but if there is, run it and start from scratch or if you're PC proficient, read the installer log and remove EVERY bit by hand first!!!