Hi Dean Technically, the window frame is the 2X material in the wall that frames in a window.....I'll bet you mean the window sill..yes? If this is the case, and the rot is not too extensive, you can get a couple of wood rasps for your drill and grind out the rotten wood (all the way back to good wood) and fill with a two part epoxy that mixes like bondo for auto body work (this is available through most big home improvement centers). Once that sets up (about 30 minutes) you can shape it down with a sure-form plane, a sharp scraper or a course sander. The alternative is to take the window all the way out and replace the sill (or whatever wooden part has rotted). This is not too difficult a job, it just takes time and patience.
The other part of this fix is to figure what's causing the rot. This is usually caused by retained moisture either due to not keeping it painted or a non-draining storm window.