Okay... I'm sorry you have problems with it. I can see over a thousand people using multiAVCHD for the last few hours...
At this point your problem hasn't occurred with other people. If you have another computer you can try there.
I have four different computers (XP Pro, XP Home, Vista Ultimate, Vista Business) and multiAVCHD works for all four. Sorry, but I can't test anymore, too.
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multiAVCHD_nwg.exe
Here is executable especially for you. If it doesn't work...I just don't know. You should see all spawned windows open/close... And .264 files in _TEMP folder will not be deleted. If they are 0 bytes - the problem will be located easier.