What is the difference between generic, ICL, and 64-bit builds?

The main difference is the compiler that is used for compiling ffdshow.ax and thereby resulting CPU requirements:

Build Bit Compiler Requirements
Generic 32 Microsoft Visual C++ Pentium II/Athlon or higher
64 Microsoft Visual C++ 64-bit CPU and 64-bit Windows
ICL 32 Intel C++ Compiler SSE2-capable CPU like Pentium 4/Athlon 64 or higher