Having your plants on the window sil is not the same as having the plant directly under the sun outdoors. Your plant will stretch to hell and probably fall over, IF it even gets that far. If you can't afford an HID, then invest in some cheap CFL's that you can buy at Home Depot.

If you do decide to try the window method, and by a miracle of God it lasts the 3 or 4 months (thats how long it will take to get some bud)....you might get a joint or 2....tops....sorry champ.