Greetings Daniel,
Just to add to Rob Jax,
You want the least abrasive cleaner necessary to do the job.
Here's a list, starting with the least and IMHO finishing with the most abrasive.
Toothpaste, Car Polish, T Cut, Brasso, Propriety scratch remover.
In all cases (excluding the worst case scenario described above ie gouge marks) you need to use small circular motions similar to how 'bull' is put on guardsmen boots and keep going over the affected area and then outwards away from the scratches. Do not try to get rid in one go, take your time aiming to use the least effective method.
If you go at it with one of the more abrasive removers, you will finish up with a window full of small circular shapes. You then go back over the window with a less abrasive polish and so on until you are left with a perfectly clear window.
Its one of those jobs that, when completed, gives a tremendous amount of job satisfaction.