import bpy

scene = bpy.context.scene
nodes = scene.node_tree.nodes

render_layers = nodes['Render Layers']
output_file = nodes.new("CompositorNodeOutputFile")
output_file.base_path = "//my/folder/"

slicepath =str(os.environ.get("basepath")

and i get S:ClientsBlender279scene1a

i think blender wants S:\Clients\Blender279\scene1a\