Which StaxRip version are you installing? I notice in the change log there's an entry about AviSynth plugin failure in 64 bit systems.
Release Name: StaxRip 126.96.36.199 beta
Fixed bug AviSynth plugins setup failing on 64bit systems
Updated x264 to version 1179
Updated mkvtoolnix to version 2.9.7
Updates x264 features reflecting recent x264 chang
I don't know how you are using StaxRip. I've tried it a few times. But if you are mainly going from DVD to AVI with no HiDef then I'd recommend trying AutoGK. Usage is a lot simpler. I do admit though I haven't tried StaxRip in a bit. I could never figure out how to get the audio to be anything other than mp3.