What you need to do is always read a string (when asking for input letter) and calculate the length of the string (to determine if it's a character or a word). Change all the scanf functions in fgets functions. Remember that the fgets function also reads the \n character in buffer.
Code:
char word[1024];
char input[1024];
...
// input word to guess
printf("imput word to guess\n ");
fgets(word, 100, stdin);
word[strlen(word)-1] = 0; /* remove \n char */
...
// input letter (or word)
printf("\n input letter\n");
fgets(input, 100, stdin);
input[strlen(input)-1] = 0; /* remove \n char */
...
if(strlen(input) > 1)
{
/* It's a word */
if(strcmp(word, input) == 0)
printf("YOU WIN\n");
else
printf("Wrong guess\n");
}
else
{
/* It's a letter. Here you can use input[0] in stead of c */
}
B.T.W. Your main should return a value!