The latest version of Defraggler that will run on Windows 2000 is v.1.21.209. Piriform has restricted Defraggler to Windows XP and above after this version. You can download Defraggler v.1.21.209 from HERE.
Support for Windows 2000 ended from Microsoft on July 13, 2010.
System requirements for each Piriform product are listed on the FileHippo and Piriform sites