Can't Specify HTML Editor After Upgrade

[gr8dane]

I recently upgraded from TP 4.7.3 to TP 5.4.2 and now I can't specify which program Windows automatically uses as HTML editor. I used to have it set to TP, but now there's nothing to select from in the box. Could it have something to do with the fact that I installed the upgrade on top of the previous version (as opposed to uninstalling the old version first)? I know there's a bunch of TP4 stuff left over in the registry. What can I do to fix this?