Use a proprietary adhesive of the No Nails™ type. Use it to fix a batten across the window opening, say 75mm deep and 20mm thickness - full width.
When the glue is set hard (24 hrs) decorate the batten and screw the pole fixings into the wood - you may have to use different screws - shorter and fatter ones than those supplied - this may mean drilling slightly larger holes in the support brackets.
When the pole and curtains are up you will hardly notice the batten.
Good luck...
Peccavi, July 2009
