As james said you tracking is most likely out, this is the cheap and easy problem to fix. But i would recommend going to a centre that has a four wheel laser alighment centre as they will be able to check everything.
Is your car lowered? If so it knocks out your camber and castor which can only be fixed by either turning the car to standard or buying some Whiteline gear.
Before anyone suggests it, it cannot be the bushes as this will make the centre point different ever time you drove the car. The worst case for this is that the n/s damper has gone and due to the o/s being stronger the car is being pulled one way. To repair it will need some knowledge and some money.
Mike