If you are getting error 0xC800021F on the Windows Update site on your Windows 2000 system, open a command line and run the following commands:
regsvr32 msxml32.dll /s
net stop wuauserv
cd /d %windir%\SoftwareDistribution
rd /s /q DataStore
net start wuauserv
This will register the MSXML DLL file correctly, then stops the Automatic Updates service (so that no temp files are locked), deletes the cache for Windows Update, and finally restarts the Updates service. After you have done this, you should be able to run the Windows Update website normally.