I had the same exact problem with leaks in a Metal Industries single hung window I installed a few years ago. I stuffed pieces of paper towl in the track to sash corner where the leaks are. I did this every winter and took the paper towl pieces out come spring. One shouldn't have to do this of course, but it worked and was easy to do.
I installed slightly more expensive vinyl windows in my porch. When the wind blows the sashes actualy flex where they meet, leaking like crazy. They whistle like haunted house sound effects. I didn't heat the porch in the winter so I didn't do anything about it.
I just had vinyl window installed in my new (to me) house. I went with the crank out awning windows. They seal just great. I read that the crank out casement type windows also seal better than double hungs do. I know this was true of the wood sash windows and vinyl sash windows I had in my old house. They leaked around the crank mechanisms instead.
My new windows don't leak around the cranks either. These new windows appear to be the best vinyl windows I have had, and they were made right here in New York State. I don't remember the brand. It is something I had never heard of before. My contractor said they are all he uses because they are good and he doesn't get complaints about them.