well there is my problem. I am making a car and I have a few things object that I have to duplicate to both sides. I cannot use the mirror modifier because there are too many pieces, therefore it would take too long.
most pieces are around the wheel and are fairly small.
I used the CTRL+M thing but ther problem is that they don’t stay at the same place.
is there a way too keep them where they are and they don’t center all on one point?
Move the centre of all your meshes to right place so when you mirrior them you are in control of where the mirror goes.
I am not sure that I understood.
but what I if you mean to move the center of the meshes one by one. there is not really a point. and if you do it all at the same place. it does the same :S.
but thanks anyway.
if you didn’t mean that could you tell me what it was. 
cars are symetrical so your centre positions would all be in the same place, its probably more me not understanding you then visa versa.
well the thing is that when you mirror something from the object mode, there is only one choice. But it does it seams to do it from the center of the object.
I was wondering if there was a way to group them and do it and ungroup them later.
but if there is none I guess I’ll do it the long way…
I think I am just a bit too lazy 
There is. I always use lattices for this kind of stuff (parent all the objects to a lattice which has its center on the line along which you need to mirror your objects), then duplicate them, the objects as well as the lattice, (or better yet, use Alt+D to make instances) and mirror one of the lattices in object mode. Then you can unparent the objects again (select all of them and hit Alt+P and use Clear Track).