A pre-assembled, spring loaded, block and tackle window sash balance assembly is adapted for providing vertical support for a window sash running in an elongated, generally upright sash run having a generally C-shaped guide channel extending along its plow region. The assembly comprises a generally U-shaped housing for a block and tackle spring balance device. The assembly further includes spaced slide means which are discrete from the housing itself and which are slidably interengageable with the guide channel for removeably attaching the assembly to the sash run. Accordingly, the block and tackle balance assembly may be pre-assembled and then conveniently and efficiently attached to the sash run and supported in a position to provide spring balanced support for a window sash mounted in the sash run.