Solved
Mathcad Express pim_installmgr_mathcad.exe dissappears
I entered registration data to obtain a download link, received the link, and downloaded the installer. When I run the installer, it starts unpacking, but at the end of the unpacking process, the installer simply closes, and that's it.
I tried extracting the installer and looking for the files mentioned on forum, but the only executable file in the installer is setup.exe.