Thread: Stop button doesnt stop a function (using PostMessage)

I'm trying to make a function that will scan through the files that are in a directory of the users choice to search for a certain file. However, the stop button I tried to make doesn't actually work. It's not even possible to do anything while it's searching, including closing the program. I'm using PostMessage() to go through the process one file at a time, hoping that after each file the user could have the option to stop it. Unlike having SendMessage where the user definitely cannot do anything until it's finished. Is there a different route to take so I can stop it in mid search?

Here is what I think is causing the problem.

Code:

        //update what the user sees
        SendMessage(hStatus, SB_SETTEXT, 0, (LPARAM)"Scanning..");

        //update percentage
        percent = 100*(sofar/total); //get the total percent

        itoa(percent,Percent,10);   //change the integer percent to string

        strcat(Percent,"%"); //add percent sign to percentage

        char OverAll[MAX_PATH] = "Progress: "; //reset the overall progress

        strcat(OverAll,Percent); //set overall percentage

        SendMessage(hStatus, SB_SETTEXT, 1, (LPARAM)OverAll); //display overall percentage

        PostMessage(Hwnd,WM_COMMAND,SCAN,0); //repeat the process

EDIT: just tried using Sleep(500) and it still has the same problem. I clicked the stop button really fast hoping I'd catch it, but nothing. I also can't regain focus of the window.