Stuck at the cabin with an old XP machine, and couldn't figure out how to update the root certificates, which were causing constant errors visiting webpages. Finally hit upon certmgr.msc, which allows viewing what certificates are available, and realized that just typing the name of a downloaded cert will prompt the wizard to install it.
Remembering the assoc and ftype commands:
C:\Documents and Settings>jc\assoc .crt .crt=CERFileC:\Documents and Settings>jc\ftype cerfile cerfile=rundll32.exe cryptext.dll,CryptExtOpenCER %1
last updated 2020-10-21 17:56:31. served from tektonic.jcomeau.com