Condensation On Inside Of Window

we are suffering at the moment with condensation on the inside of the window in our main bedroom. It is a bay window with a radiator under it; we have curtains hanging in the bay window, so when the curtains are closed the radiator is between the curtains and the window. At first I thought it may be this setup causing the problems so I turned the radiator off. However the window was still covered with condensation the next morning. The house is about 5 years old and the windows are double glazed. I had it in my head that there should be no condensation and it seems by looking around that the neighbours dont have this problem.

any opinions appreciated.

Whitey