windows fopen fails randomly with permission denied

I'm trying to read and write to files in the c drive of my computer. the fopen function fails randomly.

FILE *fp = fopen(strIn, "r");
if(fp == NULL){
    printf("Error: %d (%s)\n", errno, strerror(errno));
    getchar();

}

the weird thing in my case is , fopen fails randomly.sometimes succeeds and sometimes fails. cannot predict why.i'm always running it in admin mode.whether this could be windows issue? the error message it get is error-13 , permission denied. im running the code in administrator mode.i have tried calling

_chmod(outFile , _S_IREAD);

checked the return value and then called fopen.The fopen failed again.