Torque issues with DC motor
You could use belts and pulleys, or buy gears and try to assemble a reducer, but I think you'd be better off just buying a gear-head motor. There is no way to electronically increase the torque much beyond the rated torque- we can reduce the speed and it's possible with some effort to maintain the torque. A gear-head motor will have the reduction gear train integrated into the motor assembly. 

You need linear motion rather than rotary, which points to the use of a leadscrew (as you mention) or timing belt kind of arrangement. Calculating the torque required is more mechanical engineering but I think you can estimate it from lever arms and so on, and add a healthy safety factor to account for friction in the slides and drive train. 

You probably also should consider a safety stop so that the window does not double as a dull guillotine as well as end-of-travel limit stops, of course.