Quicktime alternative wouldn't work because it just uses the Apple Quicktime decoder but strips away the "goodies" installed with a regular install of Quicktime.
You're looking for a 64bit directshow filter which can decode Quicktime video, because Apple hasn't written/compiled a 64bit version, you'll have to look at other filters which can decode Qt. Mplayer/ffmpeg could but hasn't got a 64bit version and can not be used as a filter. But ffdshow which is an awesome decoder package does have a 64bit version, depending on the version of Quicktime your trying to decode I think it would work.
If you're trying to decode HD quicktimes you would have to wait for the CoreCodec AVC 2.0 release which has a 64bit filter..