Download from the "COCOMO™ II.99.0" link which needs to be installed to get the helpfiles.

Download from the "USC COCOMO™ II.2000.0.a" link, which points to
http://sunset.usc.edu/COCOMOII/beta_software/cocomo990920.exe.

Put that version in the same directory as the installed COCOMO™II.99.0 software, and run it instead.

You can rename it if you wish to COCOMO.exe (after renaming the installed copy to COCOMOII99-0.exe).