WINDOWS 2000 PROFESSIONAL
One of the easiest ways is to look in the registry.
Navigate to the HKEY_LOCAL_MACHINE\Software\Microsoft\Updates\Windows 2000 key, which contains registry keys for each installed update.
These keys take the name of the specific service pack or update in question. The update names originate from the Microsoft Knowledge Base article that describes the problem and the update.
If you need more information about updates, you can use the Qfecheck.exe tool, which you can download from Microsoft's Web site. In addition to pulling the information on updates from the registry, Qfecheck.exe checks the version number stored in the registry against the version number of the update file. Qfecheck.exe also checks the Windows System File Protection catalogs to verify that the cached version is the correct one.
To use Qfecheck.exe, open a command console, and type QFECHECK at the command prompt. This returns the results to the screen. To log the results to a file, use the /I switch, and specify a location.
NOTE: Editing the registry is risky, so be sure you have a verified backup before making any changes.