How to execute a snippet of code before the main execution!

Before trying to hack your OS by modifying some kernel parts to jump somewhere else, you should try the second idea you thought of. (I assume you only used the wrong words and you don't want to execute something before Windows loads the executable into memory)

Just create the process with CREATE_SUSPENDED, which will prevent it from running. Then do what ever you want to do with it either from your loader process or with CreateRemoteThread. Then you can resume the process.