OK. Solved!

Another thing which was (apparently) relevant was that, between the last time I used Avast! External Control and now, I have installed WindowsBlinds!!

With the help of Filemon from sysinternals, I saw that irsetup.exe was spinning when WindowsBlinds was attempting to skin the Irsetup UI. So, a quick unload of the WB process and the Avast! External Control Tool is working again.

It would seem that the Avast! External Control Tool extracts a small number of exectables to the TEMP folder and then attempts to run them from there. This is where irsetup.exe comes from. Adding (in my case) X:\Temp\irsetup.exe to the exclusion list in WindowsBlinds (per application settings) now fixes things so that I can run the External Control Tool without having to unload WB first!

Maybe this’ll help for anyone else who runs into the same situation, though it’d be even better if Avast! External Control used facilities which were skin aware.

Chris