How do I open windows that are painted closed?
If your windows have a little bit of play, pull down first. You can pull down much harder than you can push up, without damaging the window. Once you hear the pop of the paint freeing, you can push up normally. I'll even put all the force on one side and then another.

You'll still want to cut away any obvious locations where the paint is sticking, but if your windows are anything like mine, this problem will be a reoccurring because the paint can get into the track itself and readhere over time. I've been dealing with windows that stick for many years now, one of the gifts of a prior home owner that wasn't careful enough. The only thing I haven't done yet is to hit the windows with some silicone lubricant to see if that would help.