Some people posting to the microsoft.public.* newsgroups have reported
that Disk Cleanup's "Compressing Old Files" stage can take overnight, so you may want
to leave Disk Cleanup running that long.
Also, some people post that Disk Cleanup stops responding while compressing old
files. The following Microsoft Knowledge Base Article explains a workaround that, if
implemented, removes the option to Compress Old Files (although written for Windows
XP, the resolution also works for Windows 2000).