Firstly, yes it's the free HTML editor. Sorry I didn't mention this originally but when I started this support ticket I selected the product (free HTML editor) from a drop down list so I didn't think it needed to be mentioned again.
A point of clarification: when I launch the program, I get the error message which I can clear by clicking OK. The program is then open and looks normal but almost any interaction causes the error message to reappear and so the program is effectively unusable. But it does, as I say, open, and is not being blocked by the UAC as far as that goes. There may be other UAC issues I guess but I know little about that side of things.
Scott, I'm not sure what DEP is...
I'm using Microsoft Security Essentials for antivirus and the standard Windows firewall (no laughter please!

)
I'd forgotten about the trial version of the full program. I'll give that a go. At the moment have gone back to version 9.6 of free while I complete current project.