I think that rock vs knight is a win, if you play correctly and the knight and rook doesn't connect. Look at
http://www.everything2.com/index.pl?node=Rook%20vs%20Knight%20Endgames
But rock vs bishop? I think it is a draw. But a fork or skewer might save the day. Look at
http://www.chesskit.com/training/endgame/KRkb/1/index.php
If you have a specific position, use this homepage
http://www.logicalchess.com/resources/tablebase/
Does King and Rook win against King and Knight? Against King and Bishop?