According to my understanding INTEL_W32 here is used to define some values which corresponds to Intel Machine (Processor) and Win 32 plat form, which can be (Windows 98, Windows 2000 or Windows XP).
The reason for this thing is that any statement begins with # are preprocessor directives. Compiler process them before processing of the programme source code.
For example, #define PI 23.7 causes every occurrence of the word “PI” to be replaced with the number 23.7. All these things are done before the compiler compiles the source code and the linker links.
It is just like #include in C programming language which causes the contents of io.h processed first.
But in the case of #if defined INTEL_W32 directive, any material between the #if and a #else or #endif will be included or processed in the source code if there has been a previous statement which defines INTEL_W32. If there is no such statements which define INTEL_W32, then the block of code with in the directive will not be sompiled and processed.