When you select the Thumbnails view in Windows 2000 Explorer, JPEG files do NOT display properly.
To resolve this issue:
1. Start / Run / regsvr32 thumbvw.dll / OK.
2. Press OK to the DllRegisterServer in thumbvw.dll succeeded message.
If this doesn't fix the problem, see Upgrading Office 2000 to Office XP on Windows 2000 kills .gif, .jpeg, and .jpg thumbnail views?