Metal box section lintels are often used over windows and make curtain rail fitting difficult. Use "no nails" type of adhesive (mastic gun cartridge) to glue up a wooden batten across the window opening with an overlap each side - same length as the curtain rail you want to fit.
Next day when glue is set - emulsion paint a couple of coats and fit rail using woodscrews.
You may have to get creative to prevent batten sagging while the glue sets.
Peccavi
October 2008