There seems to be some confusion as to what is in MSDN. The bits I've
seen DO contain SOME source from NT: An assortment of drivers, for
example ATAPI (IDE CDROM), a sample SCSI miniport driver, printer drivers
et cetera. The idea is, if you've got to write a device driver, you can
see sample source. The kit that contains this is called the DDK (driver
developer's kit).
Nowhere have I seen source for memory management, scheduling or anything
like that. The larger hardware manufacturers apparently do have access to
this, as NT out of the box only supports 2 or 4 CPUs, but patches are
available from vendors (e.g. NCR) of larger SMP boxes to support all
CPUs.