Eureka! Figured out exactly what's going on. Some posters with similar issues said that right-clicking the .scr file in SysWOW64 and selecting "Install" caused it to show up. I also experimented with running the 32-bit version of the screen saver control panel (with "rundll32.exe c:\windows\SysWOW64\shell32.dll,Control_RunDLL desk.cpl,,1"). Interestingly, the 32-bit CP (control panel) was showing one of my 32-bit screensavers, which wasn't in either System32
or SysWOW64; it had been installed into the registry. I also discovered I did have a 32-bit screensaver in SysWOW64, which showed up in the 32-bit CP but not the default 64-bit CP. After right-clicking on that .scr file and selecting Install, it appeared in both control panels. Also, the 32-bit screensavers that weren't installed in the registry but were in the System32 directory did
not appear in the 32-bit CP.
It turns out the screen saver Control Panels will show anything properly installed via the registry,
plus any *.scr files in System32 (if using the 64-bit version of the CP)
or any *.scr files in SysWOW64 (if using the 32-bit version). So, the "correct" solution is to leave the 32-bit screensavers in SysWOW64, but install them into the registry. I'm sure there's a way to do the registry installation automatically during the Avast install....
Edit: After doing some more research, I've determined that the above isn't quite correct. Apparently, the "Install" option doesn't actually add anything to the registry (it just changes one entry). What's actually happening is that "Install" sets the registry entry for the
current screensaver to the selected file (and pops up the CP with the new entry selected). Both versions of the control panel will list all *.scr files in
the same directory as the currently selected screensaver, plus the System32
or SysWOW64 folder (depending on whether you ran the 64- or 32-bit version),
plus (this is the bit I missed before) the Windows folder (this is where the aforementioned "32-bit screensaver that wasn't in either System32 or SysWOW64" actually was).
So, it looks like the best place to install *.scr files (if you want it to work properly for either 32-bit or 64-bit Windows and not have 32-bit code in the 64-bit folder) is actually the Windows folder, not the System32 or SysWOW64 folders!
(And Microsoft did
not make it easy to figure this out!
)