Exactly as T-Cut says if your windows are scratched then it's serious work with specialist compounds to get them up to scratch
.
If they are just "dirty" my favourite cleaner is Barkeepers friend mixed to a paste with distilled vinegar. Put it on with a small sponge (the sponge side of one of those dishwashing green scouring pads is OK) leave to dry and buff off with some heavy duty tissue. If there's some that's difficult to shift just sprinkle with more vinegar and wipe off.
For those old enough to remember it's a bit like how the original pink Windowlene was used.
Finish off with a diluted meths/isopropyl alcohol (IPA) spray.
Andy.
PS Make sure you rinse any excess off as the great cleaning power comes from being slightly acidic (rather than alkaline like most cleaners) so doesn't do paintwork etc much good if left on there.