SweetFX is actually only a 32bit app, it will work perfectly fine on 64bit Win7, so long as the executable programme you want to use it with is only 32bit, so I'm guessing that is the problem now you are using the 64bit version of Mame.
But...there is a 64bit version of SweetFX out there, I use it on the 64bit build of the Dolphin emulator.
Google for it and see if that helps.
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