Step 1: Figure out WHY the dog is barking/jumping at the window. Could be something in the yard, a noise, wanting out (which you already said you don't think is the case here), or something else. Figure out why the dog is jumping, what are they gaining from the exercise? Do you always react a certain way when they do it? Has there ever been a time when they were encouraged to react to the window?
Step 2: Provide an alternate option for the dog to do instead of jumping at the window.
I've said it before, but it bares repeating ... Don't tell the dog what NOT to do, rather tell them what TO DO. So instead of punishing the behaviour that they are doing, figure out why they are doing that behaviour and provide them with an appropriate way to react to it rather than the negative one that they currently are doing.
For example, if you think your dog is barking at the window because there is something in the yard that they are alerting to, what would be an acceptable way for them to alert you other than jumping at the window? Would you prefer that they come to you? Sit? Lay down? Pick one action that you are comfortable with and ask that they do it instead of jumping at the window. So anticipate when they are going to go to the window and begin the routine (this does take supervision and thinking ahead ... but so does all dog training, it is not a lazy-man's activity

). If you see them approaching the window, ask them to come away from the window and do the action that you prefer (sit, down, come, etc.). Reward when they do that alternate behaviour. Keep at it, persistence and consistency is key. Every time they go to the window, distract them and ask for the alternate action. Reward when they do that rather than the window jumping. If they ever offer the alternate action instead of jumping on their own, HUGE praise and reward!
Shelties are alert dogs, they will alert. You just need to show them an acceptable way of doing so.