I am new to AC, and certainly no expert, but I can tell you that we have 3 networked computers in the office running XP and bought a new one to act as a server on a peer-to-peer set-up. The "server" runs Windows 7 and everything seems to be integrating fine. Upgrading from XP to Win 7 is a significant hassle, so you can keep the old ones with XP or spend the bucks to get all new computers running Windows 7.
