Create a dll to intercept a particular window creation msg and modify the msg parameters using WinAPI

I am currently writing an application that performs operations on other windows. I have a situation in which I need to work on the main window of an application while a modal dialog box is open. I have done this in a new version of the same application by hiding the modal dialog box and activating the main window using the WinAPI functions ShowWindowAsync and EnableWindow. Using this method in the new version, I'm able to work on the main window of the application without any restrictions, while the dialog box is open but hidden in the background. But when I use the same functions to hide the dialog box in the older version of the application, I can work on the main window but I'm unable to use the shortcut keys of the main window. I have tried everything, searched a lot on the net, but I'm unable to find a direct solution to this problem. Probably an indirect method would be to make the dialog box modeless before it is created so that I'm able to use the shortcut keys on the main window. For achieving that, I need a global system hook implemented which will 'intercept' a window creation message (for eg. WM_CREATE) when the dialog box is created (in a separate process). I then need to modify the message parameters so that the dialog box is created as a modeless dialog box, which would allow me to work on the main window and use its shortcuts after hiding the dialog box using the WinAPI functions mentioned above. I'm not a professional programmer, in fact I'm a finance person and switched to the software business as I was interested in it. So I'm not an expert in the technicalities and I need you to help me solve this problem, either by implementing this hooking mechanism or by any other method that you may know. You can use any language to write the dll for the hook, but as far as I have researched on the net, such a global system hook would be possible only in C++. I understand this information would not be sufficient enough to quote on this project, so please feel free to contact me to discuss the issue in detail. Note: Time is not an issue, but please do not quote unless you are sure that you can give me the solution.