Install the Polish language pack. Then use the following key combinations for the Polish characters:
Right-Alt + Z = ż
Right-Alt + X = ź
Right-Alt + S = ś
Right-Alt + C = ć
Right-Alt + L = ł
Right-Alt + O = ó
Right-Alt + E = ę
Right-Alt + A = ą
Right-Alt + N = ń
