Well, I thought I'd report this weirdness.
I downloaded and installed PortableApps.com Platform 10.0 plus 5 portable apps with no problems using Firefox+FlashGot+Free Download Manager.
The next day I downloaded 2 more apps using Chromium (not Chrome) + Free Download Manager. Those two apps would not install. I got the "The NTVDM CPU has encountered an illegal instruction" error each time.
I wiped the USB flash drive and tried again. PortableApps Platform installed OK but not the last two apps (which I tried first).
I deleted the two apps and downloaded them with Firefox+FlashGot+Free Download Manager instead. *THEY INSTALLED OK!*
If this post is in the wrong forum, sorry but I couldn't figure out where to put it.