A good article on 2D rotated rectangle collisions.

This article is simple, short and easy. Below is the link for the tutorial.

Tutorial - 2D Rotated Rectangles Collision Detection
http://www.ragestorm.net/tutorial?id=22


But in case you don't want to use rotated collision, what i do, is to treat rotated rectangles as a 4 points polygons and do the simple point in polygon Or polygon-polygon test.