I began having exactly the problem described by Whirly starting with the (mandatory) Windows 10 Creator upgrade - the 0xc0000020 errror. Uninstalling and reinstalling the current Firestorm viewer allowed me to get to SL, sometimes for several sessions, but eventually it would crash again. I was using McAfee Anti-Virus Plus (the single PC version). Annoying, since I have used it with no issues for over 15 years and am paid up through next July. I wrote a complaint on the dumb ("We're so glad to see you go - don't you know we save your life every day") Comments Card that pops up when you uninstall, attached the URL for this thread, but I'm sure, like all RL paper Comments Cards, it will just be thrown away. %$#@!
But after uninstalling McAfee, uninstalling and reinstalling Firestorm, the issue seems to be resolved. Totally Annoying.