The latest version of CCleaner that will run on Windows 2000 is v.3.09.1493. So far, Piriform has not restricted CCleaner to Windows XP and above. You can download CCleaner v.3.09.1493 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.