A bit of network knowledge is essential, yes. Please don't forget IPv6 in the interview.
Though first of all I'd try and find out if the interviewee is interested in learning about new technologies, and if s/he appears to fit into your team - no matter how brilliant s/he is with Windows, if s/he's not fitting into your team you'll have a problem on your hands.
For the technical side:
Ask some question in the direction of AD, like "if group A is a member of group B, and group B is a member of group C, which then has group A as a member - is that legal in AD, what problems would you expect, how would you go about fixing it?".
Authentication: "please explain the concept of Kerberos to me".
Backup&restore: "how would you go about backing up a handful of servers, various versions, incl. at least 2 versions of Exchange and SQL Server, and how would you do a bare-metal restore"
Understanding: "explain the different RAID levels, and what their down-/up-sides are"
Automation: "how would you go about automating testing&roll-out of security patches (mind different Windows Server versions)"
Security 1: "if you had to expose SharePoint to the outside world, how would you secure access, and how would you do server hardening, same for Exchange with Outlook Web Access"
Security 2: "what would be the down-/up-sides of getting all our Windows machines on the network with 802.1x, how would you go about managing the certificates"
Security 3: "please explain Teredo, and how would you go about ensuring that there are no such tunnels to outside our network"
Understanding of basic technologies: "please explain SMTP, and why you need DNS for that", "how would you choose a Windows domain and how would you integrate it with our DNS setup"
Day-to-day operations: "How would you go about ensuring that Exchange services stay available, transparent for the users, even when applying patches or when some hardware dies"
Deeper understanding of Windows internals: "explain SysWoW64, and how it works so 32bit binaries get to their 32bit DLLs" (that's a real tricky one, and full of security-relevant pitfalls!) - go grab a copy of the "The WoW effect" paper, and make a hands-on test for finding a file with certain contents hidden in either System32 or SysWoW64.