i tend to think it's not possible at all since both 5 and 7 are prime numbers and rand5 can't return 0.

edit: ok, i found a way... at least all numbers are equally probable here. not very elegant, though.
Code:

---

short int rand7()
{
	short int temp = 0;

	for (int i = 0; i < 8; i ++) {
		temp += rand5()-1;
	}

        if (temp == 32) {
                return rand7();
        }

        return temp/4+1;
}

---