My main computer is a Windows 7 Professional 32-bit system. I have found that Calibre
Portable 5.42.0 (the latest) runs fine on it. Have you tried installing that version? (You don't have to uninstall your previous one to try this.) You can get it here:
https://calibre-ebook.com/download_portable
Also, the regular version of Calibre 4.23.0 -- the final version 4 release -- also works fine on my Windows 7 system. You can get it here:
https://download.calibre-ebook.com/4.23.0/