Problem Resolved.

While Debugging I found in the disassembled code that the SendMessage() wasn't returning and throwing 'illegal memory access' Exception too.

Then I Looked upon the callback function code and found that the "CALLBACK" was missing in callback function's definition.

Now it is working on x86 Platform too. May be x64 compiler is smart enough in branching and linking the subroutines of assembly. Or what else could be the reason that the code was not only compiling successfully but also working perfectly on XP/Vista x64???

Anyways Thanks 'dc 2000'.

JAI SAYAN