RIM has apparently discontinued download and support for all versions of it's BlackBerry Desktop Software version previous to version 6.1, currently available at
this link.
I've archived previous versions here if anyone needs:
File name
Desktop Manager 6
Latest working version >>
610_b034_multilanguage
BlackBerry_Desktop_Manager_6.1_b20.exe
600_b047_multilanguage.exe
Desktop Manager 5
501_b080_multilanguage.exe
501_b080_multilanguage.exe
501_b073_english_nomediamanager.exe
5_1_.0.0_Release021_multilanguage.exe
Desktop Manager 4.2
421_b017_english.exe
Desktop_Software_v4.2_SP2_(English).exe