Preparing particle board subfloor for installing laminate?

Not completely level? Or very uneven? I think you should be ok with a plywood subfloor. I would stay away from any sort of self-leveling product. Wood is your best bet. If the floor is really uneven you will need to pull it up. Why don't you pull up the particle board first (it should be pretty easy, and then it might be easy to make the floor level if you have access to the main floor boards and joists. Then you can put down your sub-floor (3/4" ply, toungue and groove is best) and lay your laminate floor on top of that.