When you install a Service Pack, Windows 2000 remembers its' location.
When you install new or updated components, Windows 2000 will automatically get updated files from this location. You don't have to reinstall Service Packs.
You can change the location by using Regedt32 to navigate to:
Double-click the ServicePackSourcePath in the right-hand pane and set the string to a valid path, including a network distribution share.