Previous versions of Windows are plagued with the RSL, which limited the amount of space that could be consumed by a registry hive in the paged pool memory.

Windows XP eliminates this obstacle by mapping views of the registry in the cache address space. The causes even the largest hive to be charged with no more than 4MB of space.

NOTE: If you're interested, see http://msdn.microsoft.com/library/techart/msdn_manamemo.htm.