In my experience, Vista can screw up all ActiveX objects very easily. Make sure Xniff compitable with Vista, and that you've registered the DLL.
To register the DLL:
1. Press the Windows key+R. Type "cmd" (without quotes in the box and press enter).
2. Type "cd <PATH>" where <PATH> is the script folder, where the Xniff DLL is located.
3. Type "regsvr32 /u <DLLNAME> " where <DLLNAME> is the Xniff DLL. This will unregister the DLL.
4. Then finally register it again with "regsvr32 <DLLNAME>".
Hopefully it will give you some kind of error, which could be the problem.
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