Newegg has Windows 7 Home Premium for $84.99 and Windows 8.1 Pro for $124.99, promo code required. These are the OEM versions and physical media is included.
Newegg LinkIf you're still running XP I heartily endorse upgrading to Win 7, especially since MS is ending all support for XP in less than a week. I've only played with Win 8 a little bit and did not like the UI, but that's a personal preference.
