I am working with an organization in moving from Windows XP to Windows 7 for 20,000 devices. I am hoping that some of you have done a deployment of this size and would be willing to share your experience, major challenges and tips. Deployment will be done via LAN. I have overseen software rollouts to these devices however changing the OS offers up it's own set of challenges. Suggestions/answers to even simple questions such as can a user continue working while the OS change is occuring would be greatly appreciated.