Well, that was unpleasant.
The download link doesn't show the correct version until the install window, as Phoon said.
I'm on Vista 32 and had DX Pro 3.5 (just plain ol' 3.5, not the other updated versions of 3.9x etc.) installed already, and on the first try, I installed the update on top of it.
Results:
Try #1 (without uninstalling)
Created in - C:\Program Files\Stardock\DesktopX
Welcome screen borked (background doesn't appear.)
DLL Error - DXBuilder won't load (This procedure entry point DG_PngFromResource could not be located in the dynamic link library DirectGUI.dll)
Try #2 (after uninstall of 3.5 and CCleaner)
Created in - C:\Program Files\Stardock\Object Desktop\DesktopX Pro
DesktopX doesn't seem to be installed on machine
Welcome screen won't run at all (see above)
Themes run (without included wallpapers or scripts)
Widgets and gadgets run
Objects don't run
DXBuilder: All scripts removed, no script editor
Export Gadget option still available
Themes, objects, widgets location - C:\Users\Public\Documents\Stardock\DesktopX
Try #3 (after uninstall, CCleaner, reinstall of 3.5, and one more attempt to install update)
Created in - C:\Program Files\Stardock\Object Desktop\DesktopX Pro
More created in - C:\Users\Public\Public Documents\Stardock\DesktopX\
Welcome screen borked.
DLL Error DXBuilder won't load
Themes, objects won't load (same DLL error)
Existing widgets comprised of more than one object are borked, scripts don't work.
Gadgets work, obviously
Then I spent the rest of the day restoring my PC. Maybe I'm just unlucky, but this doesn't appear to work on Vista 32. Have any other Vista users tried it? Everyone seems to be on W7.