When this happens, press CTRL+ALT+DEL and start Task Manager. If Explorer.exe is not a running process, try starting it.
If this does not solve the problem, you most likely have an old version of a required DLL in %SystemRoot%.
Windows 2000 uses the same methodology to locate a DLL as windows NT. Since Explorer.exe is located in %SystemRoot%, any old DLL, located in %SystemRoot%, would be found before the correct version.
My guess is shdocvw.dll, but any of the following other files in %SystemRoot% can cause problems:
1. Press CTRL+ALT+DEL and start Task Manager.
2. Select New Task (Run) from the File menu.
3. Switch (CD) to the %SystemRoot% folder.
4. Rename the offending file to .old (rename shdocvw.dll shdocvw.old).
5. Shutdown and restart your computer.
NOTE: The newer versions of the above files will be located elsewhere on your disk.