I also run 64b system so that shouldn't be the problem.
I didn't take any special steps to get the program to work.
First I had downloaded Avast! Free Antivirus directly from avast web site (link:
http://www.avast.com/index )
then I rebooted the PC and downloaded NVDA from this link:
http://nvda.sourceforge.net/snapshots/master/nvda_snapshot_master-10319,6c0a602.exeand everything worked.
I'm not sure which install screen with google chrome check you mean though. The only options I have after running NVDA is "Install NVDA on this computer" and "Continue running". Both options read avast system tray icon context menu correctly.
The only difference is that I run clean Windows 8.1 not updated Windows 8.
You said you have several PCs with this issue, are all of them updated Windows 8.1?
It seems unlikely but it's possible that it's caused by the update.
Sorry for the pictures. The first shows opened avast gui with checked Self-defense and running NVDA next to avast in system tray.
The second shows system settings. Click Start -> type "screen reader" -> click "Use screen reader"
Make sure everything is unchecked especially in section "Other programs installed" if present.