We have very large environment with many technicians whom can join PCs to R2 domain, however, we want to shift away from XP OS, so is there a way in "Windows 2008 R2" active directory or group policy which we can set to prevent joining new PCs which have XP installed on them and only allow joining of PCs which have "Windows 7".

Hello, Thank you for your post here. I think there is not a direct settings there you may count on to restrict Windows XP clients from Windows Server 2008 R2 domain. Standard domain users can add up to 10 client computers to the domain by default. 1. You may cancel the right to join client computer on domain users and delegate the limited technicians permissions to join computers. 2. Or you may pre-create computer accounts which will be specified for Windows 7 clients.