I have a very large program which I have been compiling under visual studio (v6 then migrated to 2008). I need the executable to run as fast as possible. The program spends most
Forget micro-optimization such as what you are describing. Run your application through a profiler (there is one included in Visual Studio, at least in some editions). The profiler will tell you where your application is spending its time.
Micro-optimization will rarely give you more than a few percentage points increase in performance. To get a really big boost, you need to identify areas in your code where inefficient algorithms and/or data structures are being used. Focus on those, for example by changing algorithms. The profiler will help identify these problem areas.