I had this same problem a few weeks ago trying to open a text file.
Just thought I'd add a reply to this aswell. My code assumes the person doesn't know the file extension and adds on ".txt" but you can change that to anything you like.
#define LNAME "C:\\Documents and Settings\\" /* loadfile location */
char load_location = LNAME;
printf(" Enter file name: ");
fgets(file_name, sizeof(file_name), stdin);
/* remove newline left by fgets */
len = strlen(file_name);
if (file_name[len-1] == '\n')
file_name[len-1] = 0;
FILE *file = fopen(load_location, "r");