Well it'd be good to know what is moving the pointer, be it a mouse either ball or optical, or track-point, or a touch pad? If it's a ball mouse, try cleaning the ball and the rollers on the mouse. If it's optical, you might need a mousing surface with a more non-glossy texture. If it's track-point, they tend to refuse to go in certain directions once they've been worn in, and it takes more force than usual to get it to move in the direction you want, and it will often wander once you've stopped applying force. If it's a touch-pad, buy a cheap mouse and use that, touch-pads are a pain.
If the problem is more serious than that (like the pointer moving in a straight 45 degree angle in the opposite direction you push the mouse, or if there is any linear erroneous movement of the cursor) then it's possibly a mouse defect and you'll need to get a new device. You can find very decent ball mice for extremely cheap if you know where to look, and it's not uncommon to find good optical mice for ~$10-20.