In Windows 2000, the startup process loads the System hive, the PFN (Page Frame Number) database, NTLDR (Windows NT Loader), the kernel, the HAL (Hardware Abstraction Layer), and the boot device drivers, all of which must fit into 16 megabytes of memory.
In Windows Server 2003, with SP1 (Service Pack 1), with the /3GB switch, the startup process is limited to 110 MB. The System hive can be 32 MB. The System hive plus the HAL, kernel, and boot drivers can be 64 MB.
NOTE In an x64 Edition of Windows, the System hive can be up to one-quarter of physical memory, but cannot exceed 200 MB.