For others reading this thread, here is MikefromFrance's thread in the SimpleViewer forum.
There certainly seems to be something unique to one of your PCs which is causing the problem (preventing the 'New Gallery' button from working in both JuiceboxBuilder and svBuilder).
Having tried everything else I suggested in the forum thread above, perhaps you could try one of the following.
(1) Create a new Administrator User Account and install JuiceboxBuilder (and/or svBuilder) there.
(2) Install an earlier version of Adobe AIR (from the Archived Adobe AIR page) as the problem seemed to happen after upgrading AIR.
(3) Temporarily turn User Account Control off. This should turn off Virtual Store and ensure that the application's files are being read from their actual location (rather than from a virtual location). I realise that both PCs have UAC turned on and the applications run fine on one PC and not the other but, in the absence of knowing exactly what is causing the problem, I would try anything I could think of that might make a difference.
These suggestions are potential workarounds rather than solutions but they might at least allow you to run JuiceboxBuilder and svBuilder on your PC.