Kept getting the same Fatal Error. Removed all Windows updates, removed all versions of 5000 (18, 19, 20) and re-installed V20 removed Factorytalk *.rnl file still Fatal error. Decided to sleep on it. Next morning I right clicked the start-up Icon for RSLOGIX5000 V20 and looked carefully to see if it was compatible, no issues there then I noticed that the "run as administrator" was unchecked. I checked it and the Fatal error went away. Amazingly I am the only one on the internet that has this issue and can provide you with this solution. I spent 8 hours scowering the internet and no one has had this problem. You guys are great !!