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 COCOMOII.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).