What is this soft plastic like material that has been added to the window, and how best to remove it?
Likely "removable caulk", (perhaps "temporary caulk" - been a few years since I put some on leaky windows for the winter) in which case pulling it once you get an edge lifted up should be effective at removing it, though perhaps complicated by it being a sloppy job... 

Otherwise, just (clear?) caulk, and it might be harder to remove. Using a knife will risk scratching things up - a plastic scraping tool might be a better choice, depending on the window material. Then again, you may just have to plan on repainting.