You need a Meguires Claybar kit. Available from Halfords or any GOOD motor factor. Clean the car, dry off then apply the Quik Detailer supplied in the box and rub over the area with the clay. You only need to break off a piece of clay thats as big as a 50p piece. Keep the area lubricated with the Quik Detailer until you have rubbed off the spots.
You will then need to protect the area with wax. Once you've clayed a car completely you'll know the difference between just cleaning a car and detailing a car. The bodywork should feel like glass once its been clayed. The best way to tell if your car needs claying is to wash it, dry it then hold your hand out flat on the panels and just feel along the panel. If you feel any gritty bits on your finger tips then it means that you have bonded contaminants on your paint. The claybar will remove this.
Anyway, after claying, ideally you should wax with Meguires 1,2 and 3 Stage cleaning products. This will give you an excellent finish. I've got a silver Vectra and i've just tried the new Meguires NXT Tech Wax 2.0. Its fantastic!