There are two main types of condensation that occur on double glazed windows. Inside, on standard double glazed windows is down to two factors. Lack of continuous ventilation, eg trickle vents closed, night vent positions not use, or an existing damp problem in the house.
On energy rated windows such as the top specification 'A' rated window, condensation can for on the outside of the window during certain conditions. This is because virtually all the heat stays in your house and not enough escapes from the inner pane of glass to heat the outer one and burn the moisture off. This is not a fault, but merely indicates the superb insulating properties of the 'A' rated windows.