One of the privileges of being in the SQL Server Engine team is that we have our own operating system abstraction layer that provides virtual memory management, scheduling, synchronization, resource monitoring, exception handling and several other things. SQLOS is a user-mode OS that sits on top of the Windows kernel and provides common functionality required by the server. I am in the process of learning how to use SQLOS and how it works internally. Slava Ok's WebLog  is a great resource and I hope he keeps posting more information on it there.