<qa>  What is the effect on bone of Zoledronic Acid?This protocol is designed to compare the effect on bone of Zoledronic Acid 4 mg every 6 months when given upfront versus delayed start based on a post-baseline BMD T- Score below -2.0 SD at either the lumbar spine or total hip, or any clinical fracture unrelated to trauma, or an asymptomatic fracture discovered at the month 36 scheduled visit in stage I-IIIb postmenopausal women with hormone receptor positive breast cancer who will receive Letrozole 2.5 mg daily as an adjuvant therapy. 